feat: add support for auto-backports (elastic#4990)

.mergify.yml

@@ -0,0 +1,31 @@
+pull_request_rules:
+  - name: ask to resolve conflict
+    conditions:
+      - conflict
+    actions:
+        comment:
+          message: |
+            This pull request is now in conflicts. Could you fix it @{{author}}? 🙏
+            To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/
+            ```
+            git fetch upstream
+            git checkout -b {{head}} upstream/{{head}}
+            git merge upstream/{{base}}
+            git push upstream {{head}}
+            ```
+  - name: backport patches to 7.x branch
+    conditions:
+      - base=master
+      - label=v7.13.0
+    actions:
+      backport:
+        branches:
+          - "7.x"
+  - name: backport patches to 7.12 branch
+    conditions:
+      - base=master
+      - label=v7.12.0
+    actions:
+      backport:
+        branches:
+          - "7.12"