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.
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