Add disabled delete button to protected branches

What does this MR do?

Add's a disabled CSS style for the delete button on protected branches instead of hiding the button

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

Just need to check if the way I did was best practice or not. Still getting started on haml 😄

Why was this MR needed?

Improve UI

Screenshots (if relevant)

Before:

After:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #22358 (closed)