Merge branch 'sdejonge-mailers-css-compile-css-custom-properties' into 'master'

Compile CSS custom properties to hex values in mailers stylesheets

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/158903



Merged-by: Paul Gascou-Vaillancourt <pgascouvaillancourt@gitlab.com>
Approved-by: Paul Gascou-Vaillancourt <pgascouvaillancourt@gitlab.com>
Co-authored-by: Lukas 'ai-pi' Eipert <leipert@gitlab.com>
Co-authored-by: sdejonge <sdejonge@gitlab.com>