Move merge request routes under /-/ scope

What does this MR do?

Moves all merge request routes under /-/ scope.

Second attempt for !21126 (merged) but with #118849 (closed) in mind

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

Availability 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