Redirect deprecated snippets routes

What does this MR do?

Redirect unscoped snippet routing to a /-/ one. Now when - routing is a default one for almost a year, we can redirect old routing safely.

Examples:

https://gitlab.com/group/project/snippets => https://gitlab.com/group/project/-/snippets
https://gitlab.com/group/project/snippets/118849 => https://gitlab.com/group/project/-/snippets/118849
https://gitlab.com/snippets/118849 => https://gitlab.com/-/snippets/118849

FYI: We already did it for:

• issues => !28655 (merged)
• merge requests => !23066 (merged)

Issues:

• Global routing => #210024
• Project snippet routing => #223719 (closed)

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done