Resolves N+1 RPC call for Project#readme_url

What does this MR do?

It ports the usage of readme.path used in Project#readme_url into its own method at Repository#readme_path and caches the response

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.

What are the relevant issue numbers?

Closes #51061 (closed)