CI: automate backports

[taimoorzaeem]

Nixpkgs uses https://github.com/korthout/backport-action to automate backports via just a label. This works well. I think we should do the same.

It would make things easier:

• This action creates a PR, so the diff can be reviewed before merge.
• The changelog file has append mode enabled for merging, so there won't be a merge conflict, which would prevent the action from doing something.

From my experience, almost all backports apply cleanly and the changelog is in the right spot (assuming we had forward ported the changelog version update and backported all fixes in order).

Originally posted by @wolfgangwalther in #4281 (comment)

[wolfgangwalther]

This is purely GitHub Actions, has nothing to do with Nix.