scheduler: Update default policy to schedule origins without last update

Prior to this, we considered not to do it.  However, we do have some listers
which are not able to list origins with a last update. And we still need to be
able to schedule those origins nonetheless hence this change.

Refs. swh/infra/sysadm-environment#4971

swh/scheduler/celery_backend/recurrent_visits.py

@@ -34,8 +34,9 @@ logger = logging.getLogger(__name__)
 
 
 DEFAULT_POLICY = [
-    {"policy": "already_visited_order_by_lag", "weight": 50},
-    {"policy": "never_visited_oldest_update_first", "weight": 50},
+    {"policy": "already_visited_order_by_lag", "weight": 40},
+    {"policy": "never_visited_oldest_update_first", "weight": 40},
+    {"policy": "origins_without_last_update", "weight": 20},
 ]
 DEFAULT_DVCS_POLICY = [
     {"policy": "already_visited_order_by_lag", "weight": 49},