Remove FreeBSD 12.0 from CI

Remove FreeBSD 12.0 from CI, it has been EOL for some time.

Author: zeising

Diff for: .cirrus.yml

@@ -7,7 +7,6 @@ task:
     matrix:
       image_family: freebsd-13-0-snap
       image_family: freebsd-12-1
-      image_family: freebsd-12-0
       image: freebsd-11-3-stable-amd64-v20190801
   install_script:
     - sed -i.bak -e 's,quarterly,latest,' /etc/pkg/FreeBSD.conf