Resolve "Merge request collapse icon is tiny"

What does this MR do?

Resized the icon by removing the padding and making it also the toggle react to clicks on it.

Screenshots (if relevant)

(I also realigned then the icon after the screenshot)

Does this MR meet the acceptance criteria?

• Conform by the code review guidelines
  • Has been reviewed by a Frontend maintainer

What are the relevant issue numbers?

Closes #48495 (closed)