Remove prepend-left-2 CSS rule

What does this MR do?

This removes the .prepend-left-2 css class and replaces its usage for GitLab UI, gl-ml-2 utility class

issue #217418

File list

File	Converted
app/assets/stylesheets/framework/common.scss	✅
ee/app/assets/javascripts/epic/components/sidebar_items/sidebar_date_picker.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