Adjust cipher-negotiation.rst with compat-mode changes

Adjust cipher-negotiation.rst with compat-mode changes

This explains that 2.6 will ignore --cipher without --compat-mode and
restructures the whole paragraph to better readable.

Patch V2: Adjust grammar, use consistently "and later"

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20211213152529.3995394-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23403.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>