Use PDU not *Event in HeaderedEvent (#3073)

Requires https://github.com/matrix-org/gomatrixserverlib/pull/376

This has numerous upsides:
 - Less type casting to `*Event` is required.
- Making Dendrite work with `PDU` interfaces means we can swap out Event
impls more easily.
 - Tests which represent weird event shapes are easier to write.

Part of a series of refactors on GMSL.

clientapi/routing/directory.go

@@ -291,7 +291,7 @@ func SetVisibility(
 	}
 
 	// NOTSPEC: Check if the user's power is greater than power required to change m.room.canonical_alias event
-	power, _ := gomatrixserverlib.NewPowerLevelContentFromEvent(queryEventsRes.StateEvents[0].Event)
+	power, _ := gomatrixserverlib.NewPowerLevelContentFromEvent(queryEventsRes.StateEvents[0].PDU)
 	if power.UserLevel(dev.UserID) < power.EventLevel(spec.MRoomCanonicalAlias, true) {
 		return util.JSONResponse{
 			Code: http.StatusForbidden,