ci: add mergify rule for 3.6 backport

added mergify rules to create the backport
PR for release-3.6 branch.

Signed-off-by: Madhu Rajanna <madhupr007@gmail.com>

.mergify.yml

@@ -256,6 +256,14 @@ pull_request_rules:
       queue:
         name: default
       delete_head_branch: {}
+  - name: backport patches to release-v3.6 branch
+    conditions:
+      - base=devel
+      - label=backport-to-release-v3.6
+    actions:
+      backport:
+        branches:
+          - release-v3.6
   - name: remove outdated approvals on ci/centos
     conditions:
       - base=ci/centos