Updates styling of the merge details section in widget

What does this MR do and why?

The following has been updated in the merge details section of the merge request widget:

• Updated color of text
• Remove Merge details in every state except for the ready to merge state
• Updated padding for mobile

#368038 (closed)

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.