Use @gitlab/ui utilities on subepic issue flag

What does this MR do?

Related to #215474 (closed), follow issue from !27212 (merged).

• Remove temp utility.
• Use gitlab/ui utility classes.
• Remove unneeded flexbox utilities.
• Match icon margin with the task status element (using gl-m-2).

Screenshots

Before (mobile)	After (mobile)

Before (desktop)	After (desktop)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database 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