ci: switch to freebsd 13.1

freebsd 13.0 is out of support, switch to 13.1. It might be a good idea to
remove the minor version number from the image name, but there's not been a
response to that so far...

Discussion: https://postgr.es/m/[email protected]
Backpatch: 15-, just like the CI support

Author: anarazel

Diff for: .cirrus.yml

@@ -52,7 +52,7 @@ task:
 
   compute_engine_instance:
     image_project: $IMAGE_PROJECT
-    image: family/pg-ci-freebsd-13-0
+    image: family/pg-ci-freebsd-13-1
     platform: freebsd
     cpu: $CPUS
     memory: 2G