Resolve "Banzai::Filter::MergeRequestReferenceFilter has three N+1 query problems"

What does this MR do?

This merge request fixes 3 different N+1 query problems in Banzai::Filter::MergeRequestReferenceFilter.

Basically, I re-used the approach from Banzai::Filter::IssueReferenceFilter, and I had to add .includes(target_project: :namespace) for the 3rd N+1 query problem (this wasn't needed for issue, probably because the associated project's namespaces are automatically preloaded, but for merge requests, the association is named target_project instead of project so...).

Before	After

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

Why was this MR needed?

Does this MR meet the acceptance criteria?

• No Changelog entry needed.
• 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

Closes #30098 (closed) /cc @stanhu