RepositoryExists is always called with #gitaly_migration

What does this MR do?

Make Gitaly feature RepositoryExists opt-in instead of opt-out

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

No

Why was this MR needed?

gRPC doesn't go well with http proxies. This doesn't solve the underlying problem (it's an issue in the gRPC library) but it prevents users with proxies setup from experiencing this problem since they would need to explicitly enable this feature.

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 https://gitlab.com/gitlab-org/gitlab-ce/issues/38333