mergify: support backport-active-8, backport-active-9, backport-active-all

[v1v]

Description

What is the problem this PR solves?

Maybe this is solved already, but is there a way we can get an automatically kept up to date “backport-active-minors” label that auto backports to the set of minor releases that are still supported? I’d want this in Beats, Elastic Agent, and Fleet Server if it already exists. I’ve noticed an increase in forgotten backports because there are more releases to keep track of. If we can abstract this it’d make life easier, I think Kibana does something like this

How does this PR solve the problem?

Enable backport-active-all, backport-active-8 and backport-active-9 GitHub labels in Mergify to backport to a set of active branches in addition to the existing automation with the backport-<major>.<minor> labels. This can help with the current 6 active branches, which we have today.

See https://docs.mergify.com/workflow/actions/backport/#parameters

I can find regexes too:

But I'm not sure whether that can solve our current release model easily

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.