https://www.elastic.co/guide/en/elasticsearch/reference/master/release-notes-9.0.0.html has the release notes (the link on github is currently wrong — we'll fix it). that should get you going with any breaking changes (and use the upgrade assistant in kibana in 8.latest)
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or also older versions for testing: `curl -fsSL https://elastic.co/start-local | sh -s -- -v 7.17.27`
no need for my hack any more 😅