security/openvpn: security update to 2.6.20

Changelog: https://github.com/OpenVPN/openvpn/releases/tag/v2.6.20
Security: 549313db-3e93-11f1-8d38-7fbbe0285610
Security: CVE-2026-35058
Security: CVE-2026-40215
PR: 294714
Pull Request: https://github.com/freebsd/freebsd-ports/pull/511

(direct commit to quarterly, branches have diverged)