Update the FreeBSD CI image

FreeBSD 11.4 will be EoL on 30-Sept

Author: asomers

.cirrus.yml

@@ -1,7 +1,7 @@
 task:
   name: stable x86_64-unknown-freebsd-11
   freebsd_instance:
-    image: freebsd-11-4-release-amd64
+    image: freebsd-12-2-release-amd64
   setup_script:
     - pkg install -y curl
     - curl https://sh.rustup.rs -sSf --output rustup.sh