Move from ruby-sass to sassc-ruby

[compwron]

From the bundle output:
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

[compwron]

This still needs doing, here is the output from heroku deploy:

Post-install message from sass:
remote:
remote:        Ruby Sass has reached end-of-life and should no longer be used.
remote:
remote:        * If you use Sass as a command-line tool, we recommend using Dart Sass, the new
remote:          primary implementation: https://sass-lang.com/install
remote:
remote:        * If you use Sass as a plug-in for a Ruby web framework, we recommend using the
remote:          sassc gem: https://github.com/sass/sassc-ruby#readme
remote:
remote:        * For more details, please refer to the Sass blog:
remote:          https://sass-lang.com/blog/posts/7828841