security/openvpn: Security update 2.7.6 => 2.7.7

Changelog:
https://github.com/OpenVPN/openvpn/blob/v2.7.7/Changes.rst

PR: 298152
Pull request: https://github.com/freebsd/freebsd-ports/pull/610
Reported by: mandree (maintainer)
Approved by: osa, vvd (Mentors, implicit)
Security: CVE-2026-84732
MFH: 2026Q3

(cherry picked from commit 3d91d57f7f093d62e9f98322455b828aef2a5e0f)