GraphQL: Add missing calls_gitaly to some fields

What does this MR do?

Our CallsGitaly::Instrumentation notifies us if a field is calling Gitaly without having been marked with calls_gitaly: true.

We can see some notifications in Sentry for:

• MergeRequest#approved
• MergeRequest#approvalsLeft
• Repository#exists

This MR adds the missing calls_gitaly: true properties to the fields.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done