Refactor status-box css classes

What does this MR do and why?

This MR:

• Removes global status-box-related css classes in favour of gl utility classes.
• Deletes statusBadgeClass prop which is no longer needed now that we've refactored status badges

Screenshots or screen recordings

No visual changes

How to set up and validate locally

1. Check the milestone page badges http://127.0.0.1:3000/h5bp/html5-boilerplate/-/milestones/1
2. Check the todo badges http://127.0.0.1:3000/dashboard/todos

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.