Clean up CSS for status-box in issuables

What does this MR do?

Clean up CSS for status-box in issuables.

This selector is used in headers of Issue, Merge Request, Epic, and Milestone, and in inline of TODOs. (Note: milestone list is no more using this while originally introduced in gitlab-foss!19832 (merged))

Out of these usages, usage in the headers for Issue, Merge, Epic was replaced with .issuable-status-box for mobile, so this change needs to consider a regression in the header of Milestone and in the inline usage on TODOs.

Screenshots (strongly suggested)

before	after

No regression 1

No regression 2

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
• Code review guidelines
• Style guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #329734 (closed)