Update to dotnet sdk 6 in throughput workflow

Author: YunchuWang

.github/workflows/perf-testing-setup.yml

@@ -26,7 +26,7 @@ jobs:
     - name: Set up Dotnet 2.x
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: '3.1.405'
+        dotnet-version: '6.0.x'
     - name: Setup k6 for throughput testing
       run: |
         cd $GITHUB_WORKSPACE