Dropping support for TailwindCSS < 3

[stormwarning]

I'm considering removing the classic mode that allows this plugin to work with older versions of Tailwind.

Since the latest (and likely any future) version of Tailwind uses JIT and doesn't allow the older method of building all the styles at once, I can remove a bunch of code and ensure tests can run against the latest version. Officially, the latest version will be supported, but Tailwind v2 in JIT mode should continue to work as well.