Remove flash warning from login page

What does this MR do?

Redirects the login page to unlogged users attempting to access the root_path. It prevents a raising exception when unlogged users try to access the root_path.

Are there points in the code the reviewer needs to double check?

1. app/controllers/root_controller.rb
2. The tests included.

Why was this MR needed?

To fix an UX related issue.

Does this MR meet the acceptance criteria?

• Changelog entry added
• 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?

#25360 (closed)