CI: cirrus: switch to FreeBSD-13.0

we use outdated FreeBSD-12.2, which is outdated, let us update
to the actual release
diff --git a/.cirrus.yml b/.cirrus.yml
index 9b83e61..d4ced7e 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -1,7 +1,7 @@
 FreeBSD_task:
   freebsd_instance:
     matrix:
-      image_family: freebsd-12-2
+      image_family: freebsd-13-0
   only_if: $CIRRUS_BRANCH =~ 'master|next'
   install_script:
     - pkg update -f && pkg upgrade -y && pkg install -y openssl git gmake lua53 socat pcre