Migrate code base to Java 21

[strangelookingnerd]

Now that core is moving towards Java 21 there are quite a few improvements to the code base that can be made thanks to that.
These changes were produced by "Java Level Migration" recipes provided by IntelliJ IDE, including:

Java 5

• Reference to empty collection field replaced with method call

Java 8

• Replace if-cascades with switch
• Use Comparator combinator
• Loop replaced with Collection.removeIf()
• Statement lambda replaced with expression lambda
• Replaced lambda with method reference
• Anonymous type replaced with lambda

Java 9

• Null check replaced with method call

Java 14

• Statement replaced with enhanced switch

Java 15

• Use text blocks

Java 16

• Use pattern variable

Java 21

• Use SequencedCollection method
• Use record pattern

In case there are any questions to the particular recipe or change don't hesitate to ping me.

There sure are some more improvements to be made, such as usage of records. However I do not know how well these play with (de-)serialization and some of the mechanisms used in Jenkins / Stapler. As I do not want to risk breaking anything, this might be something to investigate in the future.
So from what I can see all changes here are internal, so no API changes or change in behavior in general - just (subjectively) cleaner code through simplifications.

I know that "cleanup" or "refactoring" PRs are generally frowned upon due to the large change set.
Yet I hope that others also see the benefit of making use of the language features that are available to us and keep the code base up to date with them.

Testing done

Tested changes locally.

Proposed changelog entries

• none

Proposed changelog category

/label internal, skip-changelog

Proposed upgrade guidelines

N/A

Submitter checklist

• The issue, if it exists, is well-described.
• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
• There is automated testing or an explanation as to why this change has no tests.
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
• New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
• UI changes do not introduce regressions when enforcing the current default rules of Content Security Policy Plugin. In particular, new or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
• For dependency updates, there are links to external changelogs and, if possible, full differentials.
• For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@jenkinsci/core-pr-reviewers

Before the changes are marked as ready-for-merge:

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
• Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
• Proper changelog labels are set so that the changelog can be generated automatically.
• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
• If it would make sense to backport the change to LTS, be a Bug or Improvement, and either the issue or pull request must be labeled as lts-candidate to be considered.

[StefanSpieker]

I really like this, but the amount of changes is quite high. Maybe split the PR into smaller pieces? You already have quite nice commits. If you would bundle them into smaller packages, it would be a lot easier to review.

[Copilot]

Pull request overview

This pull request modernizes the Jenkins codebase by applying Java 21 language features through automated IntelliJ migration recipes. The changes include adopting SequencedCollection methods, pattern matching for instanceof, switch expressions, lambda expressions, text blocks, and other Java 8-21 improvements.

Changes:

• Replaced .get(0) and .get(size()-1) with .getFirst() and .getLast() from SequencedCollection API
• Converted if-else chains to switch expressions with pattern matching
• Replaced anonymous classes with lambda expressions and method references
• Adopted pattern matching for instanceof to eliminate explicit casts
• Used text blocks for multi-line strings
• Applied Objects.requireNonNull* methods for null handling

Reviewed changes

Copilot reviewed 194 out of 194 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
war/src/main/java/executable/Main.java	Applied Objects.requireNonNullElseGet for cookie name handling
Multiple test files	Converted .get(0) to .getFirst() for SequencedCollection
core/src/main/java/jenkins/security/ApiTokenProperty.java	Converted if-else to switch expression (with duplicate comment issue)
core/src/main/java/jenkins/model/Jenkins.java	Lambda conversion and Objects.requireNonNullElseGet usage (with unreachable code bug)
core/src/main/java/hudson/Functions.java	Objects.requireNonNullElseGet usage (with unreachable code bug)
Multiple CLI files	Text blocks and switch expression conversions
Multiple core files	Pattern matching, lambda conversions, and switch expressions

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[Copilot]

Pull request overview

Copilot reviewed 194 out of 194 changed files in this pull request and generated 1 comment.

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[github-actions]

Please take a moment and address the merge conflicts of your pull request. Thanks!

[Copilot]

Pull request overview

Copilot reviewed 194 out of 194 changed files in this pull request and generated no new comments.

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[MarkEWaite]

I've reviewed it in detail and submitted several small changes that came from the review. GitHub Copilot has reviewed the changes as well and changes have been implemented based on its feedback.

This has passed BOM testing and ATH testing.

This PR is now ready for merge. We will merge it after approximately 72 hours if there is no negative feedback.

/label ready-for-merge

[Copilot]

Pull request overview

Copilot reviewed 194 out of 194 changed files in this pull request and generated no new comments.

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[MarkEWaite]

The most recent test failure was due to a Jenkins update center outage. I've scheduled a new build to start in about 6 hours. I assume that the update center outage will be resolved by then.

[A1exKH]

LGTM.