Complete migration to new "areas" API format

Depends on: https://github.com/alphagov/notifications-api/pull/3314

Previously:

- We introduced a new "areas_2" API field that's can
populate the "areas" column in the same way.

- We updated the Admin app to use the new field, so that
the old "areas" and "simple_polygons" API fields are unused.

- We repurposed the unused "areas" API field to use
the new "areas_2" format.

This PR:

- We can switch the Admin app back to the "areas" field,
but in the new format.

Future PRs:

- Remove support for the unused "areas_2" field (migration complete)

tests/__init__.py

@@ -696,7 +696,7 @@ def broadcast_message_json(
         'reference': reference,
 
         'personalisation': {},
-        'areas_2': {
+        'areas': {
             'ids': area_ids or ['ctry19-E92000001', 'ctry19-S92000003'],
             'simple_polygons': simple_polygons or [],
         },