[ML] Add automatic backport GitHub Action

[edsavage]

Summary

• Adds a GitHub Actions workflow (.github/workflows/backport.yml) that automatically creates backport PRs when a merged PR has version labels (e.g. v9.3.0)
• Uses sorenlouv/backport-github-action@v10.2.0 which reads the existing .backportrc.json configuration
• Updates .backportrc.json: fixes stale branchLabelMapping (v8.17.0 -> v9.4.0 for main) and adds current release branches (9.1, 9.2, 9.3) to targetBranchChoices

How it works

1. Add version label(s) to a PR (e.g. v9.3.0 to backport to branch 9.3)
2. When the PR merges, the GitHub Action triggers and cherry-picks the commits
3. A backport PR is automatically created targeting the mapped branch
4. Backport PRs receive the backport label (preventing recursive backport loops)

All required version labels already exist on the repo (v9.4.0, v9.3.0, v9.2.0, etc.).

Notes

• The workflow uses GITHUB_TOKEN (default GitHub Actions token). If backport PRs need to trigger CI workflows automatically, a bot PAT (e.g. elasticmachine) may be needed as a follow-up, since GITHUB_TOKEN-created events don't trigger other workflows.
• The pull_request_target trigger ensures the workflow runs in the context of the base repository.

Test plan

• Merge a test PR with a version label (e.g. v9.3.0) and verify a backport PR is created automatically
• Verify backport PRs receive the backport label
• Verify PRs without version labels don't trigger unnecessary action runs
• Verify backport PRs themselves don't trigger recursive backports

Made with Cursor

[prodsecmachine]

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