[FIX] calendar: fix partners_to_notify to have flat list of ids

Before this commit, user see a traceback:
(res_users.partner_id in (3,ARRA.. ...
operator does not exist: integer = integer[]

After update from initial commit
After append from commit 1bb74b7588
Now,  extend to allow [1].extend([2,3])

This commit closes #22944

Courtesy of @paulius-sladkevicius for reporting

addons/calendar/models/calendar.py

@@ -1415,7 +1415,7 @@ class Meeting(models.Model):
                     partners_to_notify = meeting.partner_ids.ids
                     event_attendees_changes = attendees_create and real_ids and attendees_create[real_ids[0]]
                     if event_attendees_changes:
-                        partners_to_notify.append(event_attendees_changes['removed_partners'].ids)
+                        partners_to_notify.extend(event_attendees_changes['removed_partners'].ids)
                     self.env['calendar.alarm_manager'].notify_next_alarm(partners_to_notify)
 
             if (values.get('start_date') or values.get('start_datetime') or