Move merge request routes under /-/ scope

What does this MR do?

Moves all merge request routes under /-/ scope. For #29572. This is part of large ongoing effort to move all project and group routes under /-/ so we can make routing faster and simpler.

With this MR we get closer to 50% routes moved.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style 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.