ci: Disable old GHC versions in CI to save compute minutes

Unfortunately, we're bumping into the limits that GitLab has imposed on free-tier accounts for CI minutes; we've only got 400 minutes a month, so we've got to make sure to use them carefully. Eventually we may end up migrating the CI onto CircleCI or something? Not really that important to me, I just used Gitlab CI in the first place because it was right there and easily available.