Apple's new M1 devices only work with Node 15+. Given that Node 16 is almost at the LTS release stage, I think it's safe to align our requirements with Node 16 to save Mac-using contributors from experiencing any pain.
This PR also includes a smol migration to gulp-sass@5, which was blocking us from migrating to Node 16. Bonus: the new release of gulp-sass no longer includes node-sass, so now our installation is ~14mb lighter!