Remove mini_racer and update SASS syntax

[benlk]

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These changes are intended to allow the project to be compiled in modern computer systems.

Changes:

• adds a .nvmrc to pin a specific Node version: 16
• updates Node dependencies
• upgrades Gem dependencies
  • removes mini_racer dependency as it could not compile on my Ubuntu 24.04 build server
  • pins execjs at 2.7.0 in order to fix an error in autoprefixer: https://github.com/rails/execjs/issues/99#issuecomment-988724515
  • adds the liquid gem at a specific version to work around a 'tainted' error: https://talk.jekyllrb.com/t/liquid-4-0-3-tainted/7946
  • upgrades Jekyll to 4.3.3: https://jekyllrb.com/news/2023/12/27/jekyll-4-3-3-released/
• updates division syntax in SASS files to remove the use of / for division, which is deprecated