[chore](ci) Auto cherry-pick labeled PRs to branch-4.2

[hello-stephen]

What this changes

Extend the existing master-side Auto Cherry-Pick workflow to recognize the dev/4.2.x label. Once a labeled master PR is merged (or the label is added after merge), run the existing checked script against branch-4.2 and create a cherry-pick PR for review. On conflict, use the existing cherry-pick-conflict label.

The dev/4.1.x trigger and branch-4.1 step remain unchanged. This does not auto-merge branch-4.2 PRs.

Validation

• actionlint passes with the two pre-existing action-version warnings suppressed; both warnings are also present on current master.
• git diff --check passes.
• The script SHA512 still matches the workflow pin.
• A recent 4.1 auto-pick job completed successfully, confirming the shared checkout/setup actions still run in the current environment.

Runtime 4.2 validation is possible after this workflow is merged into master, using a merged master PR carrying dev/4.2.x.

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[hello-stephen]

skip buildall

[github-actions]

PR approved by anyone and no changes requested.

[github-actions]

PR approved by at least one committer and no changes requested.