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

[v1v]

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

How to test this PR locally

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

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