[FIX] event: correctly give a template when sending badge

Oversight of odoo/odoo@d9ffbe9e89 a valid template_id was incorrectly converted
into a reference field, used in event automated communication (which is not
the case when sending badge).

Task-2488019
PR odoo/odoo#68901

addons/event/models/event_registration.py

@@ -235,7 +235,7 @@ class EventRegistration(models.Model):
             default_model='event.registration',
             default_res_id=self.id,
             default_use_template=bool(template),
-            default_template_ref='mail.template,%i' % template.id,
+            default_template_id=template.id,
             default_composition_mode='comment',
             custom_layout="mail.mail_notification_light",
         )