Move admin settings spinach feature to rspec

What does this MR do?

Replace the Spinach specs for admin settings with RSpec feature specs as per #23036 (closed)

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?

#23036 (closed)