Update mergify config for 8.6 (elastic#2265)

Author: lcawl

.mergify.yml

@@ -15,6 +15,20 @@ pull_request_rules:
             git merge <remote-repo>/{{base}}
             git push <remote-repo> {{head}}
             ```
+  - name: backport patches to 8.6 branch
+    conditions:
+      - merged
+      - base=main
+      - label=backport-8.6
+    actions:
+      backport:
+        assignees:
+          - "{{ author }}"
+        branches:
+          - "8.6"
+        title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})"
+        labels:
+          - backport
   - name: backport patches to 8.5 branch
     conditions:
       - merged