• Add step in case of UnsupportedKafkaVersionException[1] in the multi-version upgrade using OperatorHub
• In the case of multiple version upgrades using the OperatorHub, a UnsupportedKafkaVersionException[1] occurs when a new channel is selected in the OperatorHub UI and the cluster operator is upgraded, because the upgraded cluster operator doesn't support two or more older Kafka versions.
• In the case of the installation file, the "spec.kafka.version” should be changed if the error message[1] is returned, as part of the cluster operator upgrade[2].
• However in the case of OperaterHub, there is no description in the documentation of what to do when UnsupportedKafkaVersionException occurs. Even in the case of OperatorHub, the same step[2] is necessary.
• I don't know if it should be in the documentation, but there is a workaround to avoid UnsupportedKafkaVersionException by not specifying "spec.kafka.version" in the Kafka CR yaml.
• (For now, the current documentation also resolves the UnsupportedKafkaVersionException by changing "spec.kafka.version" in this step [3] of the Kafka upgrade. However, I doubt that was the intent.)

[1] UnsupportedKafkaVersionException : oc get Kafka -o yaml -n <Namespace>

- apiVersion: kafka.strimzi.io/v1beta2
  kind: Kafka
...
  status:
    conditions:
    - lastTransitionTime: "2022-05-18T13:03:48.604Z"
      message: 'Unsupported Kafka.spec.kafka.version: 2.8.0. Supported versions are: [3.0.0, 3.1.0]'
      reason: UnsupportedKafkaVersionException
      status: "True"
      type: NotReady
    observedGeneration: 1

[2] Deploying and Upgrading AMQ Streams on OpenShift > 9.4. Upgrading the Cluster Operator
    https://access.redhat.com/documentation/en-us/red_hat_amq_streams/2.1/html-single/deploying_and_upgrading_amq_streams_on_openshift/index#proc-upgrading-the-co-str

5. If the new Operator version no longer supports the Kafka version you are upgrading from, the Cluster Operator returns an error message to say the version is not supported. Otherwise, no error message is returned.

- If the error message is returned, upgrade to a Kafka version that is supported by the new Cluster Operator version:
      i.  Edit the Kafka custom resource.
      Ii. Change the spec.kafka.version property to a supported Kafka version.
- If the error message is not returned, go to the next step. You will upgrade the Kafka version later.

[3] Deploying and Upgrading AMQ Streams on OpenShift > 9.5.4. Upgrading Kafka brokers and client applications
    https://access.redhat.com/documentation/en-us/red_hat_amq_streams/2.1/html-single/deploying_and_upgrading_amq_streams_on_openshift/index#proc-upgrading-brokers-newer-kafka-str

...
3. Change the Kafka.spec.kafka.version to specify the new Kafka version; leave the log.message.format.version and inter.broker.protocol.version at the defaults for the current Kafka version.