Make repository, issues and MR route changes under `-` forwards-compatible

From #118840 (comment 262560184):

1. First release: Handle both /xxx and /-/xxx. URL helpers should still use /xxx so that this doesn't break with a rolling deploy.
2. Next release: Switch URL helpers to use /-/xxx and then switch /xxx to a redirect.

Part of huge effort #29572 (closed)

TODO

12.7

• Get !22082 (merged) merged
• Get !22092 (merged) merged

12.8

• Make merge request routes scoped by default => !23066 (merged)
• Add redirect from /merge_requests to /-/merge_requests => !23066 (merged)
• Make repository routes scoped by default => !23391 (merged)
• Make issues support both scoped and unscoped routing => !23779 (merged)

12.9

• Make issues route scoped by default. => !24791 (merged)

13.0

• Redirect old issues routing to scoped one => !28655 (merged)
• Change all occurrences of issues to use scoped path, address #208669 (closed)
• Make pipelines support both scoped and unscoped routing => !30159 (merged)

13.1

• Make pipelines route scoped by default. !30730 (merged)

13.11

• Redirect old pipelines routing to scoped one. !53990 (merged)

14.0

• Add redirect from repository routes except: raw, blob, tree and blame. => !34867 (merged)
• Rename repository.rb to repository_deprecated.rb => !63085 (merged)
• Rename repository_scoped.rb to repository.rb => !63406 (merged)

Updating leftover links in code comments and documentation

Should be handled in separate issue => #332964 (closed)

Removing deprecated routes

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