Edit 9.0.0 Kibana release notes for rc1 build

[florent-leborgne]

^^

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[mergify]

This pull request is now in conflict. Could you fix it @florent-leborgne? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch <remote-repo>
git checkout -b rns-900rc1-kibana <remote-repo>/rns-900rc1-kibana
git merge <remote-repo>/9.0
git push <remote-repo> rns-900rc1-kibana

[lcawl]

This PR was affected by #2984 so I've pushed a commit to fix the conflicts. Basically the rc1 content is in a separate source file but get published in a single page alongside the other kibana 9.0 releases.

[wajihaparvez]

Thank you! LGTM with two small comments 🚀