security/openvpn: bugfix update to v2.6.3

I separately verified the OpenVPN signature and ran more tests,
which the PR did not announce if it did.

Changelog: https://github.com/OpenVPN/openvpn/blob/release/2.6/Changes.rst#overview-of-changes-in-263
PR: 270831
MFH: 2023Q2
(cherry picked from commit 9152aca61800588efe5ebd43398f23704e325028)