Copy merge request routes to - scope

What does this MR do?

Another approach to !21126 (merged) and #118849 (closed)

Copy merge request routes under - scope so GitLab can serve both at the same time. We need this so, when we deprecate unscoped path, we can do a canary deploy without people getting 404. See !22082 (comment 262919897) for details

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content