fix: use dotnet 6.0.x for github actions

.github/workflows/dotnet-core.yml

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