Add section about common error with OpenVPN 2.6 and OpenSSL 3.0

Add section about common error with OpenVPN 2.6 and OpenSSL 3.0

We expect a number of configurations to no longer work with OpenVPN
2.6 and OpenSSL 3.0. This section tries to explain the most common
errors that will come up and how to work around them.

Patch V2: several mistakes highlighed and suggestions made by Frank
          included.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20221129113031.3735598-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25571.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>