Add ability for users to unprotect branch to Branch rules UI
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# Context
See [Slack](https://gitlab.slack.com/archives/CK75EF2A2/p1755012335455769) / https://gitlab.com/gitlab-org/gitlab/-/merge_requests/201176
In the [Protected branches API](https://docs.gitlab.com/api/protected_branches/), and Protected branches UI, we allow users to Unprotect a protected branch and specify which roles, users, and groups can unprotect a branch. However, this ability is not present in Branch rules.
# Design proposal
issue