added version of dotnet

Author: FabianGosebrink

.github/workflows/dotnetcore.yml

@@ -12,6 +12,6 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 2.2.108
+        dotnet-version: '3.1.x'
     - name: Build with dotnet
       run: dotnet build --configuration Release