Add `robots.txt` to the list of reserved namespaces

What does this MR do?

Reserves 'robots.txt' to prevent conflicts with the NGINX configuration

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

Why was this MR needed?

Although it's unlikely that a user will try to create a namespace with this name, if they do, it will be broken. robots.txt is handled by static routing before reaching Rails.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• 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?

See #22759