implement mergify rules for release branches (#10135)
* implement mergify rules for release branches
We only handled the case of backports previously, but the current
release checklist expects that we can commit PRs to release
branches during a release (e.g. changelogs, because we want the
list of changelog.d files that are actually part of the release).
* block merging if PR has a 'blocked:' label
* update backports strategy for #10260
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Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>