prepare 9.0 and 8.x

[v1v]

Motivation/summary

• Bump version 9.0.0
• Prepare docker files to use 9.0.0
• Enable mergify to 8.x
• Add backport-8.x to the new PRs targeting main

Tasks

• Use pinned version, likely in a follow-up since, it requires those upstream projects to create those versions...
  • But it's required to merge this before so.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Related issues

Part of #13968

[mergify]

This pull request does not have a backport label. Could you fix it @v1v? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.

[mergify]

This pull request is now in conflicts. Could you fix it @v1v? 🙏
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 feature/prepare-9.0.0 upstream/feature/prepare-9.0.0
git merge upstream/main
git push upstream feature/prepare-9.0.0

[v1v]

I plan to merge this PR sometime tomorrow