[Release] update version to 8.9

[AndersonQ]

Update the version to 8.9.0

Merge as soon as it's green

[mergify[bot]]

This pull request does not have a backport label. If this is a bug or security fix, could you label this PR @AndersonQ? 🙏. For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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#### Build stats * Start Time: 2023-04-28T11:41:42.264+0000 * Duration: 19 min 7 sec

:grey_exclamation: Flaky test report

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