[FW][FIX] payment: pass amount_to_refund as a kwarg

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closes odoo/odoo#138918

Forward-port-of: odoo/odoo#138460
Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>

addons/payment/models/payment_transaction.py

@@ -311,7 +311,7 @@ class PaymentTransaction(models.Model):
             raise ValidationError(_("Only confirmed transactions can be refunded."))
 
         for tx in self:
-            tx._send_refund_request(amount_to_refund)
+            tx._send_refund_request(amount_to_refund=amount_to_refund)
 
     #=== BUSINESS METHODS - PAYMENT FLOW ===#