Replace append-bottom-20 in admin/runners

What does this MR do?

This removes the append-bottom-20 css class and replaces its usage for GitLab UI gl-mb-6 utility class inside the app/views/admin/runners/show.html.haml file

This is due to the rem values of gl-mb-6, 3 * 8px = 24px

#232531

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