Resolve "Icons for approvals remaining and approvals met"

What does this MR do?

Change approvals and merge button icons. Specifically, change red cross icons for grey warning icons.

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

Why was this MR needed?

Screenshots (if relevant)

1. Not approved

2. Approved

3. Archived

4. Auto-merge failed

5. Closed

6. Conflicts

7. Failed to merge

8. Missing branch

9. Pipeline blocked

10. Pipeline failed

11. Ready to merge

12. SHA mismatch

13. Unresolved discussions

14. WIP

15. Rebase

16. Secondary geo node

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
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #4133 (closed)