Monitor Jekyll release and update to 4 when it's available.

[PaulBone]

We get the folliowing warning when setting up jekyll for the website:

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

According to this we should upgrade to sassc.

[slavfox]

Looks like Jekyll has a runtime dependency on jekyll-sass-converter, where the master has already updated to sassc, but Jekyll itself will update its dependencies with the next major release (fieldtrip/website#104 (comment), jekyll/jekyll-sass-converter#74 (comment)).

Quoting:

the thing to do here is just ignore the message and wait until Jekyll 4.0 comes out.

[PaulBone]

I'm glad I asked and didn't try to solve it myself! But now we get to look forward to a major version update..

Thanks @slavfox

I'm going to leave the bug open to remind myself to update to 4.0.

[PaulBone]

This is done.