Replace append-bottom-20 class boards

What does this MR do?

This replaces the append-bottom-20 class inside the boards app

#232531

Screenshots (strongly suggested)

Before	After with gl-mb-6 class

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