Fix build for .NET 6

Author: ascott18

.github/workflows/ci.yml

@@ -12,6 +12,10 @@ jobs:
     steps:
     - name: Checkout
       uses: actions/checkout@v2
+    - name: Install .NET SDK
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: '6.x'
     - name: Build
       run: dotnet build --configuration Release
     - name: Test