Fix FOUC when new sidebar enabled
What does this MR do and why?
Backport fix for https://gitlab.com/gitlab-org/gitlab/-/issues/417222 We enabled new sidebar by default but didn't update startupCSS fixtures. This causes a flash of unstyled content.
This feature flag is removed already in 16.2.
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This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch. -
The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes). -
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Edited by Simon Knox