Backport protected_branches_spec improvements from EE

What does this MR do?

Backports some small improvements to the protected_branches_spec that are present on EE

for context: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3089

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?