Update documentation references in systemd unit files

Update documentation references in systemd unit files

The systemd unit files for both client and server were referencing
outdated documentation as they were hard-coded to the OpenVPN 2.4.x
release branch.

Github: closes OpenVPN/openvpn#457

Change-Id: Iee289aa5df9ee0e9a03c0dc562e45dd39836e794
Signed-off-by: Christoph Schug <com+github@schug.net>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20240308140346.4058419-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28369.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit f65c656ac034a99cca09557eeb9337e7c00a7e73)