ci: test with multiple postgres versions (#13665)

- Tests now run on postgres 16 by default when run locally (can be specified with POSTGRES_VERSION)
- Adds test-go-pg-16 to test against postgres version 16
- Updates Dogfood dockerfile / nix flake to postgres version 16
- Updates docker-compose.yaml postgres tag to 16

docker-compose.yaml

@@ -25,7 +25,9 @@ services:
       database:
         condition: service_healthy
   database:
-    image: "postgres:14.2"
+    # Minimum supported version is 13.
+    # More versions here: https://hub.docker.com/_/postgres
+    image: "postgres:16"
     ports:
       - "5432:5432"
     environment: