Replace append-right-default

What does this MR do?

This removes the .append-right-default css class and replaces its usage for GitLab UI, gl-mr-3 utility class This is due to the rem values of gl-mr-3, 1 * 8px = 8px

#217418

Screenshots

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