Restore .net core sdk 3.1 in pipeline

Author: sleeuwen

.github/workflows/dotnet-core.yml

@@ -15,7 +15,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 6.0.100
+        dotnet-version: 3.1.301
     - name: Install dependencies
       run: dotnet restore
     - name: Build