options.c: update usage description of --cipher

options.c: update usage description of --cipher

GCC with -O3 complains:
warning: ‘%s’ directive argument is null [-Wformat-overflow=]

And indeed:
--cipher alg    : Encrypt packets with cipher algorithm alg
                  (default=(null)).

Since there is no real default anymore, remove it.
While here also indicate the somewhat-deprecated status
of the option.

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221228171314.133115-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25851.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>