mergify: rebase on backport PRs (backport #190)

[mergify]

Add rule to rebase on backports PRs. This also has been tested on the operator-ci-sandboxwith bootc-dev/operator-ci-sandbox#7

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This is an automatic backport of pull request #190 done by Mergify.

[mergify]

Cherry-pick of eceadf8 has failed:

On branch mergify/bp/release-0.1/pr-190
Your branch is up to date with 'origin/release-0.1'.

You are currently cherry-picking commit eceadf8.
  (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 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