Add backport action for v0.5.x branch (#775)

Author: aljo242

.mergify.yml

@@ -28,3 +28,12 @@ pull_request_rules:
       backport:
         branches:
           - release/v0.4.x
+  - name: backport patches to v0.5.x branch
+    conditions:
+      - base=main
+      - label=backport/0.5.x
+    actions:
+      backport:
+        branches:
+          - release/v0.5.x
+