CI: replace `make validate` with `Print Assumptions`

Since coqchk is not precise, we keep running into false positives when packages that we (indirectly) depend on (in particular,coq-elpi) load the standard library. Currently, this blocks the upgrade to Coq 8.20.

This patch disables make validate and instead introduces a check that runs Print Assumptions for each theorem in the prosa.results module, which are all the main results we care about.