Add index to count pending mirror updates

What does this MR do?

This adds an index to support a query from gitlab-monitor. It is rather high-frequency on GitLab.com and currently takes about 1,500ms to execute. Adding the index brings down execution time at about 1ms.

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content