[change] Replaced third-party jsonfield with Django's built-in JSONField

- Updated models to use django.db.models.JSONField
- Added migrations for check and monitoring apps
- Added migrations for sample apps used in tests

Author: pushpit kamboj

tests/openwisp2/sample_monitoring/migrations/0005_replace_jsonfield_with_django_builtin.py

@@ -3,6 +3,7 @@
 
 from django.db import migrations, models
 
+
 class Migration(migrations.Migration):
     dependencies = [
         ("sample_monitoring", "0004_alter_metric_field_name"),