Unable to edit MR with unchecked mergeability state

Summary

Unable to edit MR's target branch when mergeability state is unchecked

Steps to reproduce

• Choose an old MR which status is likely to be unchecked
• Go straight to its edit page (the mergeability check is triggered when show page of an MR is visited)
• Change the target branch
• Click save

Possible fixes

When merge status is unchecked and mark_as_unchecked is called, then the status isn't changed, because we don't allow transitions from unchecked to unchecked: https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/models/merge_request.rb#L199

Or do not trigger the marking on update if the status is unchecked: https://gitlab.com/gitlab-org/gitlab/-/blob/dfb9d338a4f232a9dcb383f509ffe8ce8247b521/app/services/merge_requests/update_service.rb#L64