ci: upgrade checkout action version (#2434)

upgrade checkout action version to fix the ci build warning

Author: WeihanLi

.github/workflows/test.yml

@@ -19,7 +19,7 @@ jobs:
     steps:
 
     - name: Check out code
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
 
     - name: Install dotnet
       run: ./build/get-dotnet.sh
@@ -33,7 +33,7 @@ jobs:
     steps:
 
     - name: Check out code
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
 
     - name: Run interop tests
       run: ./grpcweb_interoptests.sh