ci: add mergify (#361)

* ci: add mergify

* add backport for v0.11.x

.github/mergify.yml

@@ -0,0 +1,44 @@
+queue_rules:
+  - name: default
+    conditions:
+      - "#approved-reviews-by>=1"
+      - base=master
+      - label=automerge
+
+pull_request_rules:
+  - name: automerge to main with label automerge and branch protection passing
+    conditions:
+      - "#approved-reviews-by>=1"
+      - base=main
+      - label=automerge
+    actions:
+      queue:
+        name: default
+        method: squash
+        commit_message_template: |
+          {{ title }} (#{{ number }})
+          {{ body }}
+  - name: backport patches to release/v0.9.x branch
+    conditions:
+      - base=main
+      - label=backport:v0.9.x
+    actions:
+      backport:
+        branches:
+          - release/v0.9.x  
+  - name: backport patches to release/v0.10.x branch
+    conditions:
+      - base=main
+      - label=backport:v0.10.x
+    actions:
+      backport:
+        branches:
+          - release/v0.10.x
+  - name: backport patches to release/v0.11.x branch
+    conditions:
+      - base=main
+      - label=backport:v0.11.x
+    actions:
+      backport:
+        branches:
+          - release/v0.11.x