Delete Pull Request Branch after merge by default as global config

[wienans]

Feature Description

Hi guys,

we want to maintain a clean origin branch structure and devs often forget to delete the origin branch after pull request ist merged.
I found the repo option "Delete Pull Request Branch after merge by default" in the Repo but either it is not possible or i couldn't find it in the Configuration Cheat Sheet. It would be nice to have that option under repository.pull-request so it can be globally controlled as default option. https://docs.gitea.com/next/administration/config-cheat-sheet#repository---pull-request-repositorypull-request

Screenshots

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