[FIX] sale_order_dates: infinite recurssion in effective_date computation

This is due to the below revision
c1cfbf6436

Which used the `write` method on the `order` instead of assigning
the value in the variable `order.effective_date` to set the value,
as it should be done in the new API

opw-693800

addons/sale_order_dates/models/sale_order.py

@@ -46,7 +46,7 @@ class SaleOrder(models.Model):
             for pick in order.picking_ids:
                 dates_list.append(fields.Datetime.from_string(pick.date))
             if dates_list:
-                order.write({'effective_date': fields.Datetime.to_string(min(dates_list))})
+                order.effective_date = fields.Datetime.to_string(min(dates_list))
 
     @api.onchange('requested_date')
     def onchange_requested_date(self):