Removes 9.0 release notes and 9.0 rc1 upgrade docs

[KOTungseth]

Removes the 9.0 release notes and 9.0 rc1 upgrade docs from the Elastic Stack Installation and Upgrade Guide.

[github-actions]

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[mergify]

This pull request is now in conflict. Could you fix it @KOTungseth? 🙏
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 rm-90 <remote-repo>/rm-90
git merge <remote-repo>/9.0
git push <remote-repo> rm-90