replace backport github action with mergify

.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