Enable new CSS compiler by default

What does this MR do and why?

After analyzing the CSS diff between the new and old compiler, the diff doesn't have us concerned too much. We therefore want to enable the new SASS compiler in CI/CD to see if there are any failures.

Furthermore we turn it on in development by default as well

• See also: #438278 (closed)
• Link to the Diff: gitlab-org/frontend/playground/sass-compiler-diff!1 (comment 1758213709)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Restart gdk restart rails-web vite or gdk restart rails-web webpack
2. You should see CSS files compiled in app/assets/builds
3. If you add body{color: red !important} to app/assets/builds/application.css and reload the page, then a few text bits should become red. This "proves" that the new gem cssbundling is used.
4. If you now add body{color: blue !important} to app/assets/stylesheets/application.scss and reload the page, then a few text bits should become blue. This "proves" that vite/webpack started the new CSS compiler.