fix(ui)!: Remove the instance configuration option `repository.pull-request.ADD_CO_COMMITTER_TRAILERS` (was enabled by default). It was responsible for addition of unexpected trailers to commit messages in squash merges. These trailers were `Co-authored-by: ` and `Co-committed-by: `. Both used the pull request author as value, who is also assigned as the author of the squash merge commit, which they were just repeating. Furthermore, `Co-committed-by: ` is an uncommon commit trailer, and there is only one committer for a commit. The trailers were being added by Forgejo while performing the merge, bypassing user input in the UI and weren't shown in it. See further description and more examples in [#11097](https://codeberg.org/forgejo/forgejo/issues/11097).
