Update dotnet.yml

Author: miroiu

.github/workflows/dotnet.yml

@@ -16,7 +16,10 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 5.0.x
+        dotnet-version: |
+          3.1.x
+          5.0.x
+          6.0.x
     - name: Restore dependencies
       run: dotnet restore
     - name: Build