Improve importing of github pull requests

What does this MR do?

Instead of importing labels, issues and then merge requests this refactors the importer to do labels, merge requests and then issues. Because of the way GitHub data structures work this should reduce the number of API calls by the number of merge requests in the project.

Why was this MR needed?

Importing pull requests from GitHub currently is slow and requires a lot more API requests that needed. This should both speed it up and reduce the number of calls made.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• 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

What are the relevant issue numbers?

#24072 (closed)