🔧(back) remove usage of deprecated db engine

The db engine postgresql_psycopg2 does not exists anymore in django but
for BC compat it is possible to use it in the configuration and it is
replace by postgresql at runtime. We changed this settings to use the
good one.

src/backend/impress/settings.py

@@ -76,7 +76,7 @@ class Base(Configuration):
     DATABASES = {
         "default": {
             "ENGINE": values.Value(
-                "django.db.backends.postgresql_psycopg2",
+                "django.db.backends.postgresql",
                 environ_name="DB_ENGINE",
                 environ_prefix=None,
             ),