docs: Default to external-unstable ListenerClass

[sbernauer]

Description

Please add a description here. This will become the commit message of the merge request later.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

# Author
- [ ] Changes are OpenShift compatible
- [ ] CRD changes approved
- [ ] Helm chart can be installed and deployed operator works
- [ ] Integration tests passed (for non trivial changes)

# Reviewer
- [ ] Code contains useful comments
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated
- [ ] Changelog updated
- [ ] Cargo.toml only contains references to git tags (not specific commits or branches)

# Acceptance
- [ ] Feature Tracker has been updated
- [ ] Proper release label has been added

Once the review is done, comment bors r+ (or bors merge) to merge. Further information

[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