fix(lambda): use nodejs22.x as default runtime

[ahammond]

• Deprecate NODE_LATEST in favor of determineLatestNodeRuntime().
• Fix misleading description of @aws-cdk/aws-lambda-nodejs:useLatestRuntimeVersion feature flag.
• Mark nodejs18.x as deprecated (it's no longer in maintenance upstream, folks should upgrade)

Nodejs18 is no longer in maintenance: https://nodejs.org/en/about/previous-releases
and will be deprecated by AWS on 2025 September 1 https://docs.aws.amazon.com/lambda/latest/dg/lambda-runtimes.html

NODE_LATEST still points to v18:
https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/aws-lambda/lib/runtime.ts#L119

I see that there's been an issue about this since 2023: #28125

The feature flag? @aws-cdk/aws-lambda-nodejs:useLatestRuntimeVersion
https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/aws-lambda-nodejs/lib/function.ts#L182
As written, gives you NODE_LATEST (v18) by default instead of v16.

The best practice seems to be to use determineLatestNodeRuntime()
https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/aws-lambda/lib/runtime.ts#L414,
But even that can default to v18.

Issue # (if applicable)

Closes #28125

Reason for this change

I just wasted a bunch of time assuming that NODEJS_LATEST meant what it implied.
More time assuming that "useLatestRuntimeVersion" did what it said.

Description of changes

See above.

Describe any new or updated permissions being added

None.

Description of how you validated changes

Existing tests updated with new defaults.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[aws-cdk-automation]

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[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: c2bde26
• Result: FAILED
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[ahammond]

Unsurprisingly, this changes snapshots for 11 "unrelated" test suites. Before I invest more time in this, I would like to get a commitment around having this merged.

[hye-click-up]

We need this as our company need the latest support of nodejs

[trevor-clickup]

This is very odd to use latest as a deprecated version? Can we move this to something that is representative of what the upstream nodejs project is listing as a usable version?

[SimonCMoore]

We are unable to move the default from 18 currently until 20 or 22 is supported in all regions by Lambda. See #33626

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[aws-cdk-automation]

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