Use immutable WC order ID for KOMOJU sessions

WC_Order::get_order_number by default uses the order ID, but seems that
it can be changed.

* https://wp-kama.com/plugin/woocommerce/function/WC_Data::get_id

class-wc-gateway-komoju.php

@@ -197,7 +197,7 @@ public function process_payment($order_id, $payment_type = null)
             ],
             'line_items' => $line_items,
             'metadata'   => [
-                'woocommerce_order_id' => $order->get_order_number(),
+                'woocommerce_order_id' => $order->get_id(),
             ],
         ];
         $remove_nulls = function ($v) { return !is_null($v); };