WHMCS import: invoice gateway is always "none"

[Sheernox]

Env: ClientExec 7.2.2 (ionCube), PHP 8.5.10, MariaDB 11.8.9. Source WHMCS 9.

Where: plugins/import/whmcs/whmcs2ce.php line 1040. The invoice query does not select tblinvoices.paymentmethod.

Bug: Every imported invoice has gateway none, even when gateway2ce() could map the method, as it already does for clients.

Fix: Export paymentmethod and map it through gateway2ce().