chore(ci): forward port setting in the mergify.yml (#3922)

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Closes #3917

## Overview

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It modifies mergify.yml.
By adding the `forwardport:main` tag to the v1.x or v2.x branch, it can
forward-port it to the main branch.

.github/mergify.yml

@@ -15,3 +15,19 @@ pull_request_rules:
       backport:
         branches:
           - v2.x
+  - name: forward-port patches to main branch (v1.x)
+    conditions:
+      - base=v1.x
+      - label=forwardport:main
+    actions:
+      backport:
+        branches:
+          - main
+  - name: forward-port patches to main branch (v2.x)
+    conditions:
+      - base=v2.x
+      - label=forwardport:main
+    actions:
+      backport:
+        branches:
+          - main