mergify: support backport-active-branches and backport-active-minors

[v1v]

Motivation/summary

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-branches and backport-active-minors in mergify to backport to a set of active branches instead of using backport-<major>.<minor>. This can help with the current 6 active branches as 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

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

[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.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.