GitHub Actions has deprecated its Ubuntu 18.04 runners, and they will be
removed by December 1, 2022. Moreover, GitHub Actions now offers Ubuntu 22.04
runners.  It seems like a good time to upgrade our CI accordingly.

Somewhat annoyingly, the `haskell` Docker images that we use in our Dockerfiles
use such an old version of Debian that their version of `glibc` is incompatible
with any of the `what4-solvers` built for Ubuntu 20.04 or 22.04. As a result, I
switched them from the `haskell` Docker image to the `ubuntu` one. This
required some minor tweaks to how dependencies are installed, but nothing too
serious.

Fixes #1741. By upgrading the version of the solvers being used, this also
fixes #1744.

Also, delete the redundant saw.dockerfile