Further reduce N+1 on Jira pull endpoints

What does this MR do?

This builds on the optimisations in !57482 (merged) to further reduce N+1s during loading source and target project associations.

The N+1 issues happen when a collection of merge requests are serialised as API::Github::Entities::PullRequest entities.

This change also removes duplicate preload logic which was nearly made redundant after 7856804b but we can now fully remove.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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

Related to #33741 (closed)