Related merge request links copy from PipelinesPresnter

What does this MR do?

This MR provides the backend support for #58059 (moved). The frontend may now call pipeline.related_merge_request_text in the same way it calls pipeline.ref_text and display copy and links for all merge requests with a source branch matching the pipeline ref.

This doesn't full close the above issue, as there's no FE for it yet, but this MR can be merged right away and the FE changes shipped separately.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team