Make more ref RPC's mandatory

What does this MR do?

Removes the following feature flags: gitaly_find_branch, gitaly_branch_names, gitaly_tag_names, gitaly_ref_exists, gitaly_ref_exists_tags, gitaly_ref_exists_branches, gitaly_ref_delete_refs, gitaly_delete_branch, gitaly_delete_refs, gitaly_create_branch.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitaly#526 (closed), gitaly#965 (closed), gitaly#476 (closed), gitaly#474 (closed)