Convert Protected Branches feature JS code to ES6

What does this MR do?

Converts JS code for Protected Branches feature under app/assets/javascripts/protected_branches into ES6 to match with Protected Tags feature.

Why was this MR needed?

In an on-going progress of converting legacy ES5 code to ES6, this is one such attempt to clear technical debt.

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • [ ] Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

Related EE MR

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2481

What are the relevant issue numbers?

#31031 (moved)