docs: Default to external-unstable ListenerClass

[sbernauer]

Description

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1. Changes are OpenShift compatible
2. CRD changes approved
3. Helm chart can be installed and deployed operator works
4. Integration tests passed (for non trivial changes)

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1. Code contains useful comments
2. (Integration-)Test cases added
3. Documentation added or updated
4. Changelog updated
5. Cargo.toml only contains references to git tags (not specific commits or branches)

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[fhennig]

Looks good, but can you also bump the version everywhere? it still has "x.y.z-rc2" in there.

[fhennig]

and this needs to be backported to 23.4