replace backport github action with 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 add new tests to required without having access
to the github org.

An example pull request and backport can be seen here:

https://github.com/Mic92/nix-1/pull/4

and here:

https://github.com/Mic92/nix-1/pull/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.

.mergify.yml

@@ -0,0 +1,92 @@
+queue_rules:
+  - name: default
+    # all required tests need to go here
+    merge_conditions:
+      - check-success=installer
+      - check-success=installer_test (macos-latest)
+      - check-success=installer_test (ubuntu-latest)
+      - check-success=tests (macos-latest)
+      - check-success=tests (ubuntu-latest)
+      - check-success=vm_tests
+    merge_method: rebase
+    batch_size: 5
+
+pull_request_rules:
+  - name: merge using the merge queue
+    conditions:
+      - base=master
+      - label~=merge-queue|dependencies
+    actions:
+      queue: {}
+
+# The rules below will first create backport pull requests and put those in a merge queue.
+
+  - name: backport patches to 2.18
+    conditions:
+      - label=backport 2.18-maintenance
+    actions:
+      backport:
+        branches:
+          - 2.18-maintenance
+        labels:
+          - merge-queue
+
+  - name: backport patches to 2.19
+    conditions:
+      - label=backport 2.19-maintenance
+    actions:
+      backport:
+        branches:
+          - 2.19-maintenance
+        labels:
+          - merge-queue
+
+  - name: backport patches to 2.20
+    conditions:
+      - label=backport 2.20-maintenance
+    actions:
+      backport:
+        branches:
+          - 2.20-maintenance
+        labels:
+          - merge-queue
+
+  - name: backport patches to 2.21
+    conditions:
+      - label=backport 2.21-maintenance
+    actions:
+      backport:
+        branches:
+          - 2.21-maintenance
+        labels:
+          - merge-queue
+
+  - name: backport patches to 2.22
+    conditions:
+      - label=backport 2.22-maintenance
+    actions:
+      backport:
+        branches:
+          - 2.22-maintenance
+        labels:
+          - merge-queue
+
+  - name: backport patches to 2.23
+    conditions:
+      - label=backport 2.23-maintenance
+    actions:
+      backport:
+        branches:
+          - 2.23-maintenance
+        labels:
+          - merge-queue
+
+  - name: backport patches to 2.24
+    conditions:
+      - label=backport 2.24-maintenance
+    actions:
+      backport:
+        branches:
+          - "2.24-maintenance"
+        labels:
+          - merge-queue