Update dotnetcore.yml

.github/workflows/dotnetcore.yml

@@ -20,7 +20,7 @@ jobs:
     - name: Setup .NET Core SDK
       uses: actions/setup-dotnet@v1.7.2
       with: 
-        dotnet-version: 5.0.100 
+        dotnet-version: 6.0.x 
     - name: Build
       run: dotnet build  ./IoTSharp.sln  --configuration Release