Update .mergify.yml to include 8.11 branch (#537)

Author: kilfoyle

.mergify.yml

@@ -15,6 +15,20 @@ pull_request_rules:
           git merge <remote-repo>/{{base}}
           git push <remote-repo> {{head}}
           ```
+  - name: backport patches to 8.11 branch
+    conditions:
+      - merged
+      - base=main
+      - label=backport-8.11
+    actions:
+      backport:
+        assignees:
+          - "{{ author }}"
+        branches:
+          - "8.11"
+        title: "[{{ destination_branch }}] {{ title }} (backport #{{ number }})"
+        labels:
+          - backport
   - name: backport patches to 8.10 branch
     conditions:
       - merged