2f0de54ddb
We keep the latest minor release of each one of the last 3 major releases, which currently are GHC versions 8.2.2, 8.4.4, and 8.6.1. We also have ghc-HEAD, but this doesn't count. Dropping these compilers implied that we have to drop the corresponding versions of ghcjs, too. We can also drop a shitload of obsolete compiler patches that newer versions no longer need. At some point, we can probably simplify the generic builder, too.