Move repo backup RPC's to opt-out

What does this MR do?

Moves the new Gitaly RPC's used by rake gitlab:backup:... to opt-out. Our manual tests suggest these are working correctly.

If something still goes wrong, I think it's worth it to still have the feature flag. That is why I'm not making this RPC mandatory yet in this MR. If all goes well we can make them mandatory in 11.2.

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
• 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?

gitaly#749 (closed)