Remove prepend-top-4 CSS class

What does this MR do?

This removes the .prepend-top-4 css class and replaces its usage for GitLab UI, gl-mt-2 utility class

This is due to the rem values of gl-mt-2, 0.5 * 8px = 4px

#217418

File list

File	Converted
app/assets/javascripts/ci_variable_list/components/ci_variable_modal.vue	✅
app/assets/javascripts/boards/components/issue_card_inner.vue	✅
ee/app/assets/javascripts/vue_shared/metrics_reports/grouped_metrics_reports_app.vue	✅

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