Improved specificity of toggleable file headers

What does this MR do?

Improves the specificity of the diff file title toggle styling.

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

Why was this MR needed?

Currently, any .file-title will have a pointer cursor and background colour change on hover. This should be only for diff files, except discussion diff files.

Screenshots (if relevant)

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?

Closes #23147 (closed)