Make scoped snippet routing a default one

What does this MR do?

This will make snippets routes to use - scope by default but old routes are still served. As result there is no need to change every occurrence of snippets path to - one. We will do it in 14.0 when we deprecate old route and add redirect from /snippets to /-/snippets per #223719 (closed).

For #223719 (closed)

Similar to what we already did for issues !24791 (merged) and pipelines !30730 (merged)

Note: this does not affect snippets git repositories url

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