Updated diff toggle targets and added icon

What does this MR do?

Adds the new diff toggle icon and alters the toggle targets.

User can now click on the file header bar (where no other elements are above it, apart from the icon) and the Click to expand link to expand the diff and no where else.

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

Why was this MR needed?

Screenshots (if relevant)

Gif

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 you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #21369 (closed)

Closes #20326 (closed)