Update FreeBSD CI: drop 12.4 (nearly EOL) and add 14.0

12.4 is EOL as of the end of December 2023, and pkg installation will
start failing some time after that so remove those jobs now.

Author: emaste

.cirrus.yml

@@ -2,8 +2,8 @@ task:
   name: FreeBSD (shortest)
   freebsd_instance:
     matrix:
+      image_family: freebsd-14-0
       image_family: freebsd-13-2
-      image_family: freebsd-12-4
   install_script: pkg install -y gmake coreutils
   script: |
     MOREFLAGS="-Werror" gmake -j all