Add a backport bot

[silverwind]

trafficstars

Manual backporting is cumbersome when commits are cherry-pickable. I think we could use trop or similar to automatically create backports based on labels. See for example this PR.

Not entirely sure if trop is suitable, but there are other bots/actions as well for cherry-picking, like https://github.com/marketplace/actions/xealth-cherry-picker which creates PRs or https://github.com/marketplace/actions/cherry-pick-squashed-prs which directly pushes to branch.

[mscherer]

Since there is a plan to move to self-hosting gitea (as far as I know), I think the bot should be compatible with gitea

[silverwind]

Since there is a plan to move to self-hosting gitea (as far as I know), I think the bot should be compatible with gitea

If there's nothing suitable, I wouldn't oppose to just use these GitHub actions for now. Developing bots takes much time etc :)

[silverwind]

Looks this is now implemented with recent changes to Gitea bot.