Updates the quantity of one or more line items in a checkout.
This endpoint is only for updating the quantity of line items. To entirely remove a line item from the checkout, use Remove Line Items. To add a new line item to the checkout, use Add to Checkout.
This endpoint checks the amount of stock remaining for this line item. If the specified quantity
is greater than the remaining stock, then the quantity returned in the response is the total amount
of remaining stock.