Update Cirrus CI on the 0.37 branch

Remove freebsd-12, as it is no longer supported. And, update
freebsd-13 to freebsd-13-3.

.cirrus.yml

@@ -4,20 +4,7 @@
 task:
   name: stable x86_64-unknown-freebsd-13
   freebsd_instance:
-    image_family: freebsd-13-1
-  setup_script:
-    - curl https://sh.rustup.rs -sSf --output rustup.sh
-    - sh rustup.sh --default-toolchain stable -y --profile=minimal
-    - . $HOME/.cargo/env
-    - rustup default stable
-  test_script:
-    - . $HOME/.cargo/env
-    - cargo test --workspace --features=all-apis
-
-task:
-  name: stable x86_64-unknown-freebsd-12
-  freebsd_instance:
-    image_family: freebsd-12-1
+    image_family: freebsd-13-3
   setup_script:
     - curl https://sh.rustup.rs -sSf --output rustup.sh
     - sh rustup.sh --default-toolchain stable -y --profile=minimal