Change workflow branch from v2.7.x to preview

Author: liyun95

.github/workflows/preview.yml

@@ -3,7 +3,7 @@ name: preview
 on:
   push:
     branches:
-      - v2.7.x
+      - preview
   release:
     types: [released]
   workflow_dispatch:
@@ -20,7 +20,7 @@ jobs:
       - name: Check out Git repository
         uses: actions/checkout@v1
         with:
-          ref: v2.7.x
+          ref: preview
 
       - name: Extract branch name
         shell: bash