Expose Geo::ProjectRepositoryRegistry via GraphQL

What does this MR do and why?

This change adds GraphQL support for "ProjectRepositoryRegistry" as a replicable item type, following instructions from Step 3 in https://gitlab.com/gitlab-org/gitlab/-/blob/master/.gitlab/issue_templates/Geo%20Replicate%20a%20new%20Git%20repository%20type.md?ref_type=heads#step-3-implement-the-graphql-api

The code includes comments indicating this is being added for backward compatibility as GitLab prepares for a new version of project repository replication.

References

Geo: Expose Geo::ProjectRepositoryRegistry via ... (#577972)

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.