Geo - Improve the performance of slow count queries in replicator class

What does this MR do and why?

This MR changes .count queries under the Gitlab::Geo::Replicator class in favor of ::Gitlab::Database::BatchCount.batch_count to improve the performance of those queries that could be expensive on large Gitlab instances.

Identified class methods using (*)_count:

• registry_count
• synced_count
• failed_count

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #371813 (closed)