Make product customer readonly for non draft amendment lines

Closes #14727

modules/sale_product_customer/sale.py

@@ -88,6 +88,7 @@ class AmendmentLine(metaclass=PoolMeta):
             ],
         states={
             'invisible': Eval('action') != 'line',
+            'readonly': Eval('state') != 'draft',
             })
 
     @fields.depends('line')