Always use forecast quantity to compute supply on stock first

#11955 (closed) adds the supply option stock_first. For that it needs to compute the stock quantity. For now it uses the minimal between the stock quantity and the forecast without date. So this means that if the forecast is greater than the actual quantity, only the actual quantity is used and a purchase request will be created. This creates more purchases than needed to fulfill the orders which is the opposite of the goal of this option.

I think it should always use the forecast quantity to minimize the supplied quantity and in case of shortage stock_supply will create the missing purchase request and modules like sale_stock_quantity and stock_quantity_issue will help prevent issues.