[main](backport #2806) [Release] 8.9.0 changelog

[mergify]

This is an automatic backport of pull request #2806 done by Mergify.
Cherry-pick of 71625b6 has failed:

On branch mergify/bp/main/pr-2806
Your branch is up to date with 'origin/main'.

You are currently cherry-picking commit 71625b6.
  (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:
	new file:   changelog/8.9.0.asciidoc
	new file:   changelog/8.9.0.yaml
	deleted:    changelog/fragments/1682696415-crossbuild.yaml
	deleted:    changelog/fragments/1683024407-API-default-error-is-now-500.yaml
	deleted:    changelog/fragments/1684151814-fix-action-seq-no.yaml
	deleted:    changelog/fragments/1684437851-Expose-prometheus-metrics.yaml
	deleted:    changelog/fragments/1685985303-file-delivery.yaml
	deleted:    changelog/fragments/1686780651-reuse-error-channel.yaml
	deleted:    changelog/fragments/1686781749-Add-elastic-api-version-header-support.yaml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .buildkite/pipeline.yml

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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[jlind23]

buildkite test this

[elasticmachine]

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• Start Time: 2023-07-26T08:31:03.479+0000

• Duration: 39 min 24 sec

Test stats 🧪

Test	Results
Failed	0
Passed	751
Skipped	1
Total	752

💚 Flaky test report

Tests succeeded.

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