Copy project snippet routes under - scope

What does this MR do?

Copy project snippet routes so those are available at both /-/ scope and without one. Keep UI to use unscoped routes for one release.

Issue #223719 (closed)

The next step (next release) we do a switch and make UI use new group/project/-/snippets route. Then we can deprecate the old routing later.

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