Update ci.yml

Author: VisualBean

.github/workflows/ci.yml

@@ -26,7 +26,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 3.1.x
+        dotnet-version: '6.0.x'
     - name: Restore dependencies
       run: dotnet restore
     - name: Build