fix postgres exclusions in version matrix

Author: bckohan

.github/workflows/test.yml

@@ -66,6 +66,15 @@ jobs:
           - django-version: '3.2'
             postgres-version: 'latest'
 
+          - postgres-version: '9.6'
+            django-version: '5.0'
+          
+          - postgres-version: '12'
+            django-version: '5.1'
+
+          - postgres-version: '9.6'
+            django-version: '5.1'
+
           - python-version: '3.8'
             django-version: '5.1'
           - python-version: '3.9'