Merge branch 'psimyn-16-1-stable-ee-patch-07456' into '16-1-stable-ee'

Fix FOUC when new sidebar enabled

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/126783



Merged-by: Ezekiel Kigbo <3397881-ekigbo@users.noreply.gitlab.com>
Approved-by: Ezekiel Kigbo <3397881-ekigbo@users.noreply.gitlab.com>
Co-authored-by: Simon Knox <simon@gitlab.com>

config/feature_flags/development/remove_startup_css.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/407140
 milestone: '15.11'
 type: development
 group: group::project management
-default_enabled: false
+default_enabled: true