Remove warnings if mtl ≥ 2.2.1

Recent versions of `mtl` have deprecated the `Control.Monad.Error` module in favor of `Control.Monad.Except`, which causes `scotty` to output a lot of warnings if compiled with `mtl` ≥ 2.2.1. These changes simply use the `MIN_VERSION` macro to determine the most up-to-date version to use.