DeleteMergedBranchesService should not delete protected branches

What does this MR do?

Exclude the protected branches to be deleted when deleting all the merged branches.

Why was this MR needed?

In !11000 (merged) DeleteBranchService was modified, what caused DeleteMergedBranchesService to also delete protected branches.

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

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#34553