Change font size of merge details

What does this MR do and why?

• Adds the merge details header to merge blocked states
• Updates font size of merge details text to be smaller

Screenshots or screen recordings

How to set up and validate locally

• View a blocked merge request

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.

#376031 (closed)