Put backport automation in place accross repositories

[hiddeco]

To help facilitate backporting as described in #3254, without too much human effort. We need to be able to backport things in an automated manner.

Based on experiments in https://github.com/hiddeco/test-cherry-pick-automation, I have found a GitHub Action workflow configuration which would allow us to backport pull requests by labeling them.

⚠️ In the example workflow there I used cherry-pick: as a label prefix, but backport: seems to better describe what the label is doing, and should be used instead.

💡 I am trying to see if this pairs well with the declarative definition of labels, which I ran as an experiment in getsops/community.

• source-controller
• kustomize-controller
• helm-controller
• notification-controller
• image-automation-controller
• image-reflector-controller
• flux2