Replace prepend-left-10 class

What does this MR do?

This removes the prepend-left-10 css class and replaces its usage for GitLab UI gl-ml-3 utility class

This is due to the rem values of gl-ml-3, 1 * 8px = 8px

Since this rule does not comply with our design system guidelines, I "floored" the measurement to the closest one that did comply

#217418

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