Merge branch 'leipert-switch-to-css-bundling' into 'master'

Switch from sassc to cssbundling gem

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/145930



Merged-by: Stan Hu <stanhu@gmail.com>
Approved-by: Phil Hughes <me@iamphill.com>
Approved-by: Peter Leitzen <pleitzen@gitlab.com>
Approved-by: Stan Hu <stanhu@gmail.com>
Reviewed-by: Lukas 'ai-pi' Eipert <leipert@gitlab.com>
Reviewed-by: Paul Gascou-Vaillancourt <pgascouvaillancourt@gitlab.com>
Co-authored-by: Peter Leitzen <pleitzen@gitlab.com>
Co-authored-by: Lukas Eipert <leipert@gitlab.com>