Fix some issues with the backport / forwardport workflow

[arthurschreiber]

Description

Adds back the backport workflow with some improvements:

• Use the vitess-bot token
• Clean up logic

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

AI Disclosure

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.91%. Comparing base (749fe54) to head (6ee7904).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18954      +/-   ##
==========================================
+ Coverage   69.88%   69.91%   +0.02%     
==========================================
  Files        1610     1610              
  Lines      215431   215659     +228     
==========================================
+ Hits       150551   150770     +219     
- Misses      64880    64889       +9     

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[promptless]

📝 Documentation updates detected!

Updated existing suggestion: Document automated backport and forwardport workflow

[arthurschreiber]

There are a few writes to $GITHUB_OTPUT that are unused and that I think can all be removed.

[promptless]

📝 Documentation updates detected!

Updated existing suggestion: Add changelog entry for GitHub Actions automation

[timvaillancourt]

LGTM, caveat: it's unclear what changed exactly, so I've only glanced for obvious problems

[promptless]

📝 Documentation updates detected!

Updated existing suggestion: Document automated backport and forwardport workflow