Drop support for old (pre-Edo) protocols

I intend to emit UNPAIR n from the compiler very soon, so dropping support for pre-Edo versions is convenient for me.

I don't think of any good reason people should need pre-Edo support going forward. If they want pre-Edo code (e.g. for Mi-cho-coq verification, until Mi-cho-coq fully supports Edo) they can just use an old version.