Add mergify workflow for v5.1.x. (backport #1788) (#1789)

Co-authored-by: vincentwschau <99756290+vincentwschau@users.noreply.github.com>

.github/mergify.yml

@@ -69,3 +69,28 @@ pull_request_rules:
       backport:
         branches:
           - release/protocol/v4.x
+  - name: backport to release/indexer/v5.x branch
+    conditions:
+      - base=main
+      - label=backport/indexer/v5.x
+    actions:
+      backport:
+        branches:
+          - release/indexer/v5.x
+  - name: backport to release/protocol/v5.x branch
+    conditions:
+      - base=main
+      - label=backport/protocol/v5.x
+    actions:
+      backport:
+        branches:
+          - release/protocol/v5.x
+  - name: backport to release/protocol/v5.1.x branch
+    conditions:
+      - base=main
+      - label=backport/protocol/v5.1.x
+    actions:
+      backport:
+        branches:
+          - release/protocol/v5.1.x
+