Use a different Sass implementation (Ruby Sass is end-of-life)

[noahbrenner]

When running $ docker-compose up --build (or $ docker-compose run frontend rake d:setup, where I first saw it), the following message is included in the output:

Post-install message from sass:

Ruby Sass has reached end-of-life and should no longer be used.

* If you use Sass as a command-line tool, we recommend using Dart Sass, the new
  primary implementation: https://sass-lang.com/install

* If you use Sass as a plug-in for a Ruby web framework, we recommend using the
  sassc gem: https://github.com/sass/sassc-ruby#readme

* For more details, please refer to the Sass blog:
  https://sass-lang.com/blog/posts/7828841

TL;DR: We should probably switch to using the sassc gem: https://github.com/sass/sassc-ruby