do not test django 6 on postgres other than latest

Author: bckohan

.github/workflows/test.yml

@@ -93,6 +93,10 @@ jobs:
           - python-version: '3.14'
             psycopg-version: 'psycopg2'
 
+          - postgres-version: '9.6'
+            django-version: '6.0a1'
+          - postgres-version: '12'
+            django-version: '6.0a1'
           - python-version: '3.9'
             django-version: '6.0a1'
           - python-version: '3.10'