Replace prepend-top-20 in ci variables

What does this MR do?

This replaces the prepend-top-20 class in the ci variables section inside the group ci variables settings section #287688

Screenshots (strongly suggested)

Before	After with gl-mt-6

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