[main](backport #36128) docs: Prepare Changelog for 8.9.0

[mergify]

This is an automatic backport of pull request #36128 done by Mergify.
Cherry-pick of b1b4d8b has failed:

On branch mergify/bp/main/pr-36128
Your branch is up to date with 'origin/main'.

You are currently cherry-picking commit b1b4d8b785.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   CHANGELOG.next.asciidoc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.asciidoc
	both modified:   libbeat/docs/release.asciidoc

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[elasticmachine]

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• Start Time: 2023-07-27T16:11:50.932+0000

• Duration: 10 min 31 sec

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[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
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 upstream
git checkout -b mergify/bp/main/pr-36128 upstream/mergify/bp/main/pr-36128
git merge upstream/main
git push upstream mergify/bp/main/pr-36128

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)