Add badge-pill class on group member count

What does this MR do?

Adding .badge-pill does set the background on the group's member count as expected.

Screenshot

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #60026 (closed)