sass: Using / for division outside of calc() is deprecated

[Sjors]

When running master @ 667555a in Docker / Podman:

docker run --rm -v "$PWD":/site -p 4000:4000 corewebsite
Configuration file: /site/_config.yml
            Source: /site
       Destination: /site/_site
 Incremental build: enabled
      Generating... 
DEPRECATION WARNING: Using / for division outside of calc() is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div($doc-line-height, $doc-font-size) or calc($doc-line-height / $doc-font-size)

More info and automated migrator: https://sass-lang.com/d/slash-div

...

WARNING: 23 repetitive deprecation warnings omitted.

A partial fix is in 34afa34. It's very simple, but (probably) requires #1248 on the Debian CI (and real production).

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We probably don't need to upgrade to Dart Sass 2.0 anytime soon, so it's mainly about the noisy warning itself.