Create mergify config for 8.x backports (#3856)

Author: pierrehilbert

.mergify.yml

@@ -246,3 +246,16 @@ pull_request_rules:
         labels:
           - "backport"
         title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"
+  - name: backport patches to 8.x branch
+    conditions:
+      - merged
+      - label=backport-v8.x
+    actions:
+      backport:
+        assignees:
+          - "{{ author }}"
+        branches:
+          - "8.x"
+        labels:
+          - "backport"
+        title: "[{{ destination_branch }}](backport #{{ number }}) {{ title }}"