package/openvpn: security bump version to 2.5.6

Switched _SITE to https and _SOURCE to .gz because upstream does not
provide a .xz tarball anymore.

Fixes CVE 2022-0547, changelog:
https://github.com/OpenVPN/openvpn/blob/release/2.5/Changes.rst



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>