Upgrade FreeBSD's CI image to release 11.3

.cirrus.yml

 task:
   name: stable x86_64-unknown-freebsd-11
   freebsd_instance:
-    image: freebsd-11-2-release-amd64
+    image: freebsd-11-3-release-amd64
   setup_script:
     - pkg install -y curl
     - curl https://sh.rustup.rs -sSf --output rustup.sh