security/openvpn: update to 2.6.8 bug-fix release

hopefully fixes...

PR: 275206
Changelog: https://github.com/OpenVPN/openvpn/blob/v2.6.8/Changes.rst#overview-of-changes-in-268
MFH: 2023Q4
(cherry picked from commit f6ef06771b5a341e91ea38b0d758c4cf614f1b3c)