Fix channel finalizer logic (knative-extensions#3295)

Signed-off-by: Calum Murray <cmurray@redhat.com>

control-plane/pkg/reconciler/channel/channel.go

@@ -64,6 +64,9 @@ const (
 	// TopicPrefix is the Kafka Channel topic prefix - (topic name: knative-messaging-kafka.<channel-namespace>.<channel-name>).
 	DefaultDeliveryOrder         = contract.DeliveryOrder_ORDERED
 	NewChannelIngressServiceName = "kafka-channel-ingress"
+	// TopicPrefix is the old Kafka Channel topic prefix - we keep this constant so that deleting channels shortly after upgrading
+	// does not have issues. See https://github.com/knative-extensions/eventing-kafka-broker/issues/3289 for more info
+	TopicPrefix = "knative-messaging-kafka"
 )
 
 type Reconciler struct {
@@ -449,7 +452,7 @@ func (r *Reconciler) finalizeKind(ctx context.Context, channel *messagingv1beta1
 
 	topicName, ok := channel.Status.Annotations[kafka.TopicAnnotation]
 	if !ok {
-		return fmt.Errorf("no topic annotated on channel")
+		topicName = kafka.ChannelTopic(TopicPrefix, channel)
 	}
 	topic, err := kafka.DeleteTopic(kafkaClusterAdminClient, topicName)
 	if err != nil {