replace backport github action with mergify (backport #11321)

[mergify]

The current backport action cannot automerge because the github action bot does not trigger github CI actions. Mergify instead does not have this limitation and can also use a merge queue.

On top we have now a declarative configuration to allow contributers to mark new tests as required without having access to the github org.

An example pull request and backport can be seen here:

Mic92#4

and here:

Mic92#5

To complete the setup the mergify app must be enabled for this repository. It's already installed in the nixos organization for nixos-hardware and other repositories. Just go to
the settings for this repository and click on "GitHub Apps"

Motivation

Context

Priorities and Process

Add 👍 to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.

---

This is an automatic backport of pull request #11321 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 80f20fa has failed:

On branch mergify/bp/2.21-maintenance/pr-11321
Your branch is up to date with 'origin/2.21-maintenance'.

You are currently cherry-picking commit 80f20fa4c.
  (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)

Changes to be committed:
	new file:   .mergify.yml

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by them: .github/workflows/backport.yml

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