net/kamailio: Update 6.0.2 => 6.0.3

Changelog:
https://www.kamailio.org/pub/kamailio/6.0.3/ChangeLog

Release Notes:
https://www.kamailio.org/w/2025/09/kamailio-v6-0-3-released/

Replace PORTVERSION with DISTVERSION in MASTER_SITES and WRKSRC.

PR: 289852