mergify: add backport label (backport #25050)

[mergify]

This is an automatic backport of pull request #25050 done by Mergify.

Cherry-pick of 164bb5e has failed:

On branch mergify/bp/7.x/pr-25050
Your branch is up to date with 'origin/7.x'.

You are currently cherry-picking commit 164bb5ed42.
  (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:   .mergify.yml

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

To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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[botelastic]

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[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #25064 opened

• Start Time: 2021-04-14T08:12:54.245+0000

• Duration: 12 min 49 sec

• Commit: bc56c13

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