Update tests.yml

Author: CME64

.github/workflows/tests.yml

@@ -23,7 +23,7 @@ jobs:
       - name: Setup .NET Core
         uses: actions/setup-dotnet@v1
         with:
-          dotnet-version: 2.2
+          dotnet-version: '3.1.x'
       - name: use this action, test solution dir
         if: ${{ !cancelled() }} 
         uses: zyborg/dotnet-tests-report@v1