security/openvpn: security update to 2.5.6

Changelog:
https://github.com/OpenVPN/openvpn/blob/release/2.5/Changes.rst#overview-of-changes-in-256

Somewhat related to and obsoletes:
PR: 262626
Security: 45a72180-a640-11ec-a08b-85298243e224
Security: CVE-2022-0547
Security: https://community.openvpn.net/openvpn/wiki/CVE-2022-0547
MFH: 2022Q1