Set `design_management_flag` feature to default enabled

What does this MR do?

Enables the design_management_flag feature by default.

The issue ticket https://gitlab.com/gitlab-org/gitlab-ee/issues/12600 has more detail.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance 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