[8.17] (backport #367) github-actions: support backport labels after being merged

[mergify]

Support backports after a PR has been merged and labelled with backport-active-*.

Fixes a corner case when the PRs have been labeled after being merged with the supported list of active branches.

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This is an automatic backport of pull request #367 done by Mergify.

[mergify]

Cherry-pick of 6f75475 has failed:

On branch mergify/bp/8.17/pr-367
Your branch is up to date with 'origin/8.17'.

You are currently cherry-picking commit 6f75475.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   .github/workflows/backport-active.yml

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally