backfill doesn't work if our earliest known event is a redaction

This issue has been migrated from [#13675](https://github.com/matrix-org/synapse/issues/13675).

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When we attempt to backfill a room, we try to find suitable backward extremities to backfill from. To do this, we look at the [successors](https://github.com/matrix-org/synapse/blob/v1.66.0rc2/synapse/handlers/federation.py#L391) of the backwards extremities (which should be events that are known to us), and check that they are in a "visible" part of the timeline.

However, if that known event is a redaction (whose original event we haven't seen), this algorithm fails, and we skip that backfill point.

This has much the same cause, and solution, as https://github.com/matrix-org/synapse/issues/12795: we should not be filtering out redactions here.