security/openvpn: security update to 2.6.16

Changelog: https://github.com/OpenVPN/openvpn/blob/v2.6.16/Changes.rst#overview-of-changes-in-2616
MFH: 2025Q4
Security: 17a40d76-c3fd-11f0-b513-0da7be77c170
Security: CVE-2025-13086
(cherry picked from commit f0181a6845139e268922d522241c161a43e229d0)