ci: update FreeBSD image to 13.4

FreeBSD 13.4 was recently released, and that means the version
of the image used by this job (13.2) will be out of support soon.

Update it before the job starts failing because packages are no
longer compatible or the image gets retired by the provider since
it is now EOL.

.cirrus.yml

@@ -9,7 +9,7 @@ freebsd_task:
     DEFAULT_TEST_TARGET: prove
     DEVELOPER: 1
   freebsd_instance:
-    image_family: freebsd-13-2
+    image_family: freebsd-13-4
     memory: 2G
   install_script:
     pkg install -y gettext gmake perl5