When we consider whether to accept events, we should not accept those which
depend on rejected events for their auth events.

This (together with earlier changes such as
https://github.com/matrix-org/synapse/pull/10771 and
https://github.com/matrix-org/synapse/pull/10896) forms a partial fix to
https://github.com/matrix-org/synapse/issues/9595. There still remain code
paths where we do not check the `auth_events` at all.