Better specify old object slots removal policy

The comments seem to imply we can remove deprecated slots after 2.1 is
released. This is not quite true, if we still want to support upgrading
straight from 2.0. So let's specify better that they can be removed only
after upgrading from 2.0 can be deprecated.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>