resource/gitlab_branch_protection: Support `no one` in `unprotect_access_level`

This value was simply missing, the truth of allowed ones is here: https://gitlab.com/gitlab-org/gitlab/blob/d2b99cadd0204030bf789ea1523b4e02c9f2535b/app/models/concerns/protected_ref_access.rb#L12

Closes: #1267 (closed)