Label a PR 'backport <branch>' (e.g. 'backport 2.18') and, once it
merges, this workflow cherry-picks the PR's commits onto that branch
and opens the backport PR automatically. Labeling an already-merged
PR triggers it retroactively. On cherry-pick conflict the action
comments on the original PR so the backport can be done by hand.
Vinícius Ferrão