Refactor git controllers

What does this MR do?

In #39176 (closed) we're allowing resources other than projects to have repositories attached. Therefore, it doesn't make a lot of sense to have git requests handled by controllers in the Projects namespace. This MR introduces the new namespace Repositories and changes the routes for git actions.

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