drop alert_receive_channel.restricted_at column (#2646)

# What this PR does

Drop `alert_receive_channel.restricted_at` column

## 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)

---------

Co-authored-by: Vadim Stepanov <vadimkerr@gmail.com>

engine/apps/alerts/migrations/0028_drop_alertreceivechannel_restricted_at.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.2.20 on 2023-07-26 06:41
+
+from django.db import migrations
+import django_migration_linter as linter
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("alerts", "0027_remove_alertreceivechannel_restricted_at_from_state"),
+    ]
+
+    operations = [
+        linter.IgnoreMigration(),
+        migrations.RunSQL(
+            sql="ALTER TABLE alerts_alertreceivechannel DROP COLUMN restricted_at",
+            reverse_sql="ALTER TABLE alerts_alertreceivechannel ADD COLUMN restricted_at datetime(6) NULL"
+        )
+    ]

engine/apps/alerts/models/alert_receive_channel.py

@@ -198,12 +198,6 @@ 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: 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 = [
             models.UniqueConstraint(