`mqtt-kafka` does not limit client sharding to `mqtt v5`

[jfallows]

Describe the bug
When serverRef is specified in mqtt-kafka, this allows the mqtt-kafka binding to make use of kafka topic sharding and redirect clients to their zilla instance affinity, but mqtt v5 introduced the redirect capability so mqtt v3.1.1 clients cannot follow the redirect.

To Reproduce
Steps to reproduce the behavior:

1. Use mqtt.kafka.broker in zilla-examples with multiple Zilla instances
2. Ensure mqtt-sessions topic has multiple partitions
3. Update zilla.yaml to specify serverRef in mqtt-kafka binding
4. Connect with mqtt v3.1.1 client
5. See error when not on affinity Zilla instance for client id

Expected behavior
When both mqtt v3.1.1 and mqtt v5 are supported, kafka topic sharding should be disabled.

Screenshots
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Additional context
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