Deprecate --ecdh-curve with OpenSSL 3.0 and adjust mbed TLS message

Deprecate --ecdh-curve with OpenSSL 3.0 and adjust mbed TLS message

OpenSSL 3.0 deprecates SSL_CTX_set_tmp_ecdh() in favour of
SSL_CTX_set1_groups(3). We already support the SSL_CTX_set1_groups
using the --tls-groups. Adjust both mbed TLS and OpenSSL 3.0 to
say that --ecdh-curve is ingored and --tls-groups should be used.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Max Fillinger <maximilian.fillinger@foxcrypto.com>
Message-Id: <20211019183127.614175-7-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22999.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>