docs: Prepare Changelog for 9.0.1

[elastic-vault-github-plugin-prod]

Prepare Changelog for 9.0.1.

Merge once the BC that will be published is decided.

[kilfoyle]

LGTM! 🚢

I'll copy these into the Markdown docs.

[pierrehilbert]

Yes sorry for the extra step, we will work to improve that and generate directly some Markdown in the future.

[kilfoyle]

No problem at all! Just for reference, the Markdown version is here: #44184

[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 prepare-changelog-9.0.1 upstream/prepare-changelog-9.0.1
git merge upstream/9.0
git push upstream prepare-changelog-9.0.1

[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 prepare-changelog-9.0.1 upstream/prepare-changelog-9.0.1
git merge upstream/9.0
git push upstream prepare-changelog-9.0.1