[8.x] prepare 9.0 and 8.x (backport #13978)

[mergify]

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

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This is an automatic backport of pull request #13978 done by [Mergify](https://mergify.com).