Use .NET 6 for building

.github/workflows/ci.yml

@@ -16,6 +16,11 @@ jobs:
     steps:
     - uses: actions/checkout@v2
 
+    - name: Setup .NET
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: '6.0.x'
+
     - name: Set up Python ${{ matrix.python }}
       uses: actions/setup-python@v2
       with: