UpDropEventReferenceSHAPrevEvents
... as they could fail if there are duplicate events in `roomserver_previous_events`. This fixes the migration by trying to combine the `event_nids` if possible (same room) as mentioned by @kegsay in https://github.com/matrix-org/dendrite/pull/3083#discussion_r1195508963
reference_sha
Companion PR to https://github.com/matrix-org/gomatrixserverlib/pull/383