Resolve "Implement GraphQL MergeRequestsResolver"

What does this MR do?

Add field mergeRequests for project in GraphQL

What are the relevant issue numbers?

Closes #56485 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug

Closes #56485 (closed)