Move project snippets route under - scope

Timeline for snippet routes for #29572 (closed)

13.2

• Copy project snippet routes under /-/ scope. !35022 (merged)

13.3

• Make /-/snippets a default route for UI. !36091 (merged)

14.0

• Redirect /snippets to /-/snippets. Except raw. !54461 (merged)

Removing deprecated routes

Should be handled in separate issue => #28848 (closed)