Update CI to .NET 6.0.x

Author: hounsell

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
       - name: Install .NET SDK
         uses: actions/setup-dotnet@v1
         with:
-          dotnet-version: "5.0.x"
+          dotnet-version: "6.0.x"
 
       - name: Add MSBuild to PATH
         uses: microsoft/setup-msbuild@v1.0.2