remove alert_receive_channel.restricted_at unused column (#2632)

# What this PR does

## Which issue(s) this PR fixes

## 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/migrations/0027_remove_alertreceivechannel_restricted_at.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.2.20 on 2023-07-25 09:47
+
+from django.db import migrations
+import django_migration_linter as linter
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('alerts', '0026_auto_20230719_1010'),
+    ]
+
+    operations = [
+        linter.IgnoreMigration(),
+        migrations.RemoveField(
+            model_name='alertreceivechannel',
+            name='restricted_at',
+        ),
+    ]

engine/apps/alerts/models/alert_receive_channel.py

@@ -198,9 +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: remove this field after AlertGroup.is_restricted change has been released
-    restricted_at = models.DateTimeField(null=True, default=None)
-
     class Meta:
         constraints = [
             models.UniqueConstraint(