Upgrade @gitlab/ui to v12

What does this MR do?

Integration MR for gitlab-ui!1291 (merged)

• Upgraded @gitlab/ui to v12.0.0.
• Migrated GlNewButton to GlButton.

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done