CI: drop the FreeBSD 12.X build

Cirrus' new free tier won't let us have many builds, so drop the
nonessential ones. The FreeBSD 13.X build will still give us the most
relevant FreeBSD coverage.

.cirrus.yml

@@ -46,9 +46,6 @@ freebsd_task:
     - name: FreeBSD 13.2
       freebsd_instance:
         image_family: freebsd-13-2
-    - name: FreeBSD 12.4
-      freebsd_instance:
-        image_family: freebsd-12-4
 
   env:
     CIRRUS_CLONE_DEPTH: 10