CI: update FreeBSD version

According to Cirrus [1], the current available images are: 13.3, 14.0
and 15.0-SNAP). Let's use the latest stable version. Hence, new tools
are covered by the CI.

[1] https://cirrus-ci.org/guide/FreeBSD/

.cirrus.yml

@@ -105,7 +105,7 @@ task:
 task:
   name: FreeBSD
   freebsd_instance:
-    image_family: freebsd-13-2
+    image_family: freebsd-14-0
   setup_script:
     - pkg install -y autoconf automake gmake libevent libtool pkgconf
   env: