ref(notifications): remove db constraint for user column on notificationsetting table (#60736)

Getting ready to drop the table by removing foreign key constraints,
following the steps here:
https://develop.sentry.dev/database-migrations/#deleting-tables

migrations_lockfile.txt

@@ -9,5 +9,5 @@ feedback: 0003_feedback_add_env
 hybridcloud: 0009_make_user_id_optional_for_slug_reservation_replica
 nodestore: 0002_nodestore_no_dictfield
 replays: 0003_add_size_to_recording_segment
-sentry: 0607_drop_externalactor_actorid
+sentry: 0608_notification_setting_db_constraint
 social_auth: 0002_default_auto_field

src/sentry/migrations/0608_notification_setting_db_constraint.py

@@ -0,0 +1,39 @@
+# Generated by Django 3.2.23 on 2023-11-28 23:45
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations
+
+import sentry.db.models.fields.foreignkey
+from sentry.new_migrations.migrations import CheckedMigration
+
+
+class Migration(CheckedMigration):
+    # This flag is used to mark that a migration shouldn't be automatically run in production. For
+    # the most part, this should only be used for operations where it's safe to run the migration
+    # after your code has deployed. So this should not be used for most operations that alter the
+    # schema of a table.
+    # Here are some things that make sense to mark as dangerous:
+    # - Large data migrations. Typically we want these to be run manually by ops so that they can
+    #   be monitored and not block the deploy for a long period of time while they run.
+    # - Adding indexes to large tables. Since this can take a long time, we'd generally prefer to
+    #   have ops run this and not block the deploy. Note that while adding an index is a schema
+    #   change, it's completely safe to run the operation after the code has deployed.
+    is_dangerous = False
+
+    dependencies = [
+        ("sentry", "0607_drop_externalactor_actorid"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="notificationsetting",
+            name="user",
+            field=sentry.db.models.fields.foreignkey.FlexibleForeignKey(
+                db_constraint=False,
+                null=True,
+                on_delete=django.db.models.deletion.CASCADE,
+                to=settings.AUTH_USER_MODEL,
+            ),
+        ),
+    ]

src/sentry/models/notificationsetting.py

@@ -61,11 +61,19 @@ class NotificationSetting(Model):
     )
     scope_identifier = BoundedBigIntegerField(null=False)
     target_id = HybridCloudForeignKey(
-        "sentry.Actor", db_index=True, unique=False, null=True, on_delete="CASCADE"
+        "sentry.Actor",
+        db_index=True,
+        unique=False,
+        null=True,
+        on_delete="CASCADE",
     )
     team_id = HybridCloudForeignKey("sentry.Team", null=True, db_index=True, on_delete="CASCADE")
     user = FlexibleForeignKey(
-        settings.AUTH_USER_MODEL, null=True, db_index=True, on_delete=models.CASCADE
+        settings.AUTH_USER_MODEL,
+        null=True,
+        db_index=True,
+        on_delete=models.CASCADE,
+        db_constraint=False,
     )
     provider = BoundedPositiveIntegerField(
         choices=(