Run tests using .NET 6 and .NET Core 3.1

Author: aloneguid

Diff for: .github/workflows/release.yml

@@ -25,7 +25,9 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 6.0.x
+        dotnet-version: |
+          3.1.x
+          6.0.x
     - name: Install dependencies
       run: dotnet restore src/Config.Net.sln
     - name: Build