[FW][FIX] sale: method call over self instead of precise record

Each order does necessarily have the same confirmation template.
And `_get_confirmation_template` is `ensure_one` so it raises when self holds multiple records.

closes odoo#128743

Forward-port-of: odoo#128551
Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>

addons/sale/models/sale_order.py

@@ -928,7 +928,7 @@ def _send_order_confirmation_mail(self):
         :return: None
         """
         for order in self:
-            mail_template = self._get_confirmation_template()
+            mail_template = order._get_confirmation_template()
             order._send_order_notification_mail(mail_template)
 
     def _send_payment_succeeded_for_order_mail(self):