modify alert_receive_channel migration (#2634)

# What this PR does

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

engine/apps/alerts/models/alert_receive_channel.py

@@ -198,8 +198,11 @@ class AlertReceiveChannel(IntegrationOptionsMixin, MaintainableObject):
     rate_limited_in_slack_at = models.DateTimeField(null=True, default=None)
     rate_limit_message_task_id = models.CharField(max_length=100, null=True, default=None)
 
-    # TODO: drop this field in a subsequent release
-    restricted_at = models.DateTimeField(null=True, default=None)
+    # TODO: in a subsequent release, write a manual django migration to run the following
+    # migrations.RunSQL(
+    #  sql = "ALTER TABLE alerts_alertreceivechannel DROP COLUMN restricted_at",
+    #  reverse_sql = migrations.RunSQL.noop
+    # )
 
     class Meta:
         constraints = [