Don't create permanent redirect when renaming a namespace

What does this MR do?

This MR removes the concept of a permanent redirect.

Now all redirects will live as long as the old route isn't taken. When it's taken, the redirect is remove and the push/pull or request is directed straight to the new route.

It also drops the indexes created in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16211

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

backend part of https://gitlab.com/gitlab-org/gitlab-ce/issues/41758