Done tracking with multiple branches

[silverwind]

I think the bot needs to track "done" status per target branch, in go-gitea/gitea#30193, it opened a second port backport to 1.21 after we closed the first one. Maybe backport/v1.21/done labels, or search existing PRs to do it label-less.

[yardenshoham]

I agree

[silverwind]

Could likely be done by checking by existance of branch in the bot's repo:

• backport-29314-v1.21

[yardenshoham]

#125