Use `woocommerce_valid_order_statuses_for_payment` in `pay_action` too

Even though it was possible to filter the statuses in my-order.php and other places,
it was not possible in the actual pay action.
Introduce `woocommerce_valid_order_statuses_for_payment_complete` filter

Since it's possible to filter the valid order statuses for payments and cancels,
it only makes sense to be able to do the same when checking which statuses
are valid for completing a payment.