mergify: support backport label for 9.0 branch

[v1v]

Description

What is the problem this PR solves?

As we doing it in new way using github action, we just add support for 9.0 now and notification for backport-active labels.
Implementation for backport-active labels will be done in follow up PR right after that one will be merged. Like it was done for elastic-agent: elastic/elastic-agent#7814

How does this PR solve the problem?

Updating mergify config for backporting to 9.0 branch using label.

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Closes #

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

[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-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[mergify]

This pull request is now in conflict. 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/mergify-support-backport-active-minors upstream/feature/mergify-support-backport-active-minors
git merge upstream/main
git push upstream feature/mergify-support-backport-active-minors

[ev1yehor]

@v1v I removed part for backport-active labels, we can merge it and I will create follow up for it using gh action.

[ev1yehor]

@elastic/obs-docs

[v1v]

@ev1yehor, since you removed a few commits, could you revisit the PR title and description to reflect what you're actually doing?

[ev1yehor]

@elastic/obs-docs

[bmorelli25]

Thank you