drop CI testing on FreeBSD 12.0

It is no longer possible to upgrade packages on FreeBSD 12.0 now that it is out
of support [0]. More information at [1, 2].

  [0]: https://www.freebsd.org/security/unsupported.html
  [1]: https://github.com/cirruslabs/cirrus-ci-docs/issues/632
  [2]: https://github.com/cirruslabs/cirrus-ci-docs/issues/625