Add mirroring from master to main. (#102843)

.ci.yaml

@@ -6,7 +6,6 @@
 # More information at:
 #  * https://github.com/flutter/cocoon/blob/main/CI_YAML.md
 enabled_branches:
-  - main
   - master
   - flutter-\d+\.\d+-candidate\.\d+
 

.github/workflows/mirror.yml

+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Mirror master to main branches in the flutter repository.
+on:
+  push:
+    branches:
+      - 'master'
+
+jobs:
+  mirror_job:
+    permissions:
+      pull-requests: write
+    runs-on: ubuntu-latest
+    if: ${{ github.repository == 'flutter/flutter' }}
+    name: Mirror master branch to main branch
+    steps:
+      - name: Mirror action step
+        id: mirror
+        uses: google/mirror-branch-action@c6b07e441a7ffc5ae15860c1d0a8107a3a151db8
+        with:
+          github-token: ${{ secrets.FLUTTERMIRRORINGBOT_TOKEN }}
+          source: 'master'
+          dest: 'main'