Update design of suggestion diff

What does this MR do and why?

• Makes suggestion options small
• Removes extra borders on old/new lines
• Removes border radius on old lines
• Reduces padding on header

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to merge request
2. Add suggestion on diff

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #364574 (closed)