Add gl-button to admin/users dir

What does this MR do?

This adds the gl-button class to the buttons in the app/views/admin/users directory per our larger effort to move away from bootstrap

#231193 (closed)

Screenshots

Before	After

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