Replace prepend-top-15 css class

What does this MR do?

This removes the .prepend-top-15 css class and replaces its usage for GitLab UI, gl-mt-5 utility class

This is due to the rem values of gl-mt-5, 2 * 8px = 16px

Since this rule does not comply with our design system guidelines, I "ceiled" the measurement to the closest one that did comply

#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