hub: add missing Django migration

... to avoid the following warning while applying Django migrations:
```
Running migrations:
  No migrations to apply.
  Your models in app(s): 'hub' have changes that are not yet reflected in a migration, and so won't be applied.
  Run 'manage.py makemigrations' to make new migrations, and then re-run 'manage.py migrate' to apply them.
```

Fixes: commit 870d081
Resolves: openscanhub/openscanhub#319

Author: kdudka

kobo/hub/migrations/0006_alter_task_canceled_by.py

@@ -0,0 +1,21 @@
+# Generated by Django 4.2.22 on 2025-06-30 17:35
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+        ('hub', '0005_add_task_canceller'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='task',
+            name='canceled_by',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='canceled_by1', to=settings.AUTH_USER_MODEL),
+        ),
+    ]