Merge branch '41041-undefined-method-new_project_guidelines_html-unable-to-load-from-cache' into 'master'

Resolve "undefined method `new_project_guidelines_html', unable to load from cache"

Closes #41041

See merge request gitlab-org/gitlab-ce!15878

(cherry picked from commit 2750d7e6)

3b860069 Clears the current Appearance cache otherwise it breaks since…