[8.x](backport #40716) mergify: support for backport-major.minor

[mergify]

Proposed commit message

We have agreed to use backport-<major>.<minor> GitHub labels rather than backport-v<major>.<minor>.0 so the same GH label strategy works for the ingest and observability projects.

I decided to keep the current minor versions with the support for both labels.

Checklist

• My code follows the style guidelines of this project
• 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.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Author's Checklist

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This is an automatic backport of pull request #40716 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 3a5cd59 has failed:

On branch mergify/bp/8.x/pr-40716
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit 3a5cd592d2.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .mergify.yml

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[botelastic]

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