Add lower path index to redirect_routes

What does this MR do?

Adds an index for searching lowercased redirects_routes paths. Would be used ~20 times per second on Gitlab.com.

Why was this MR needed?

To improve performance of GitLab.com.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #34638 (closed)