Rename design management feature flag

What does this MR do?

This MR changes a feature flag name to be different from the EEP license feature name, in order to avoid this bug https://gitlab.com/gitlab-org/gitlab-ce/issues/64468 and allow us to enable the Design Management feature for individual projects.

This will allow us to roll out the feature to two projects first, as in our feature rollout plan https://gitlab.com/gitlab-org/gitlab-ee/issues/12600.

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