Vertically center text in MR widget buttons

What does this MR do?

Adds some CSS styles that change the buttons in the merge request widget to use flexbox display and align their items to the center.

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

Why was this MR needed?

See #42746 (closed).

Screenshots (if relevant)

Before

After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

#42746 (closed)