docs: deprecate virtio-crypto

We have outstanding guest exploitable bugs in the code which is fairly
complex and needs some serious love to clean it up. As off-CPU
cryptography acceleration seems to be waning in popularity lets just
the deprecate the sub-system with a view to removing it in a couple of
cycles.

We can also remove the backend code at that point as well.

Link: https://gitlab.com/qemu-project/qemu/-/work_items/3625
Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20260824-fixes-for-11-2-v2-7-352c6b890402@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>