Migrate Git::Repository#write_ref to Gitaly

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

If we need new specs?

No need for gitaly gem update since we're already ahead of what we need for this RPC.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes gitaly#784 (closed)