Recalculate billing rows for 10 days (prev. 4)

This effectively reverts [^1], which was only a temporary change.
I suspect the performance problem will go away with [^2].

While we've clearly been managing without this change, it resulted
in several rows being left as incorrect when letter receipts were
delayed. It makes sense for us to run this task for the same period
as we do to aggregate statuses, as status affects billing.

[^1]: e5c76ffda7
[^2]: https://github.com/alphagov/notifications-api/pull/3542

app/celery/reporting_tasks.py

@@ -25,7 +25,7 @@ def create_nightly_billing(day_start=None):
     else:
         # When calling the task its a string in the format of "YYYY-MM-DD"
         day_start = datetime.strptime(day_start, "%Y-%m-%d").date()
-    for i in range(0, 4):
+    for i in range(0, 10):
         process_day = (day_start - timedelta(days=i)).isoformat()
 
         create_nightly_billing_for_day.apply_async(