Set `design_management` feature to default enabled

https://gitlab.com/gitlab-org/gitlab-ee/issues/12600

ee/app/models/ee/project.rb

@@ -586,7 +586,7 @@ module EE
       # on prem installs yet.
       lfs_enabled? &&
         feature_available?(:design_management) &&
-        ::Feature.enabled?(:design_management_flag, self) # Named to avoid the bug: https://gitlab.com/gitlab-org/gitlab-ce/issues/64468
+        ::Feature.enabled?(:design_management_flag, self, default_enabled: true)
     end
 
     def design_repository

ee/changelogs/unreleased/12600-enable-design-management-feature.yml

+---
+title: Added new Design Management feature for GitLab Premium. With Design Management,
+  you can upload design assets to issues and view them all together to easily share
+  and collaborate with your team.
+merge_request: 14582
+author:
+type: added