Drop CVC4 support

[RyanGlScott]

The CVC4 repo has been deprecated and unmaintained for many years, and the CVC5 developers now recommend switching to CVC5 instead of using CVC4. We have kept a CVC4 support in Cryptol for many years due to its previous popularity, but it is becoming increasingly difficult for end users to even build or install CVC4 locally (see also GaloisInc/what4-solvers#57). I think it is time that we officially drop CVC4 from Cryptol and only support CVC5 going forward.

[RyanGlScott]

Aside from simply removing the ability to do :set prover=cvc4 (and the code paths associated with this option), we would also need to migrate any existing test cases that rely on cvc4 to use cvc5 instead. See #1805 for one challenge that arises when migrating the tests.

[sauclovian-g]

This might be a good thing to announce in the upcoming release and then do right after cutting said release.

(the actual deprecation part; no reason to wait to update tests)