Bump setup-dotnet to v2 and use .net 6

Bump actions/setup-dotnet@v2 and bump dotnet version to 6.x

This should help with the current build issues.

Author: AlexMacocian

.github/workflows/dotnetcore.yml

@@ -14,9 +14,9 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Setup .NET Core
-      uses: actions/setup-dotnet@v1
+      uses: actions/setup-dotnet@v2
       with:
-        dotnet-version: 3.1.101
+        dotnet-version: '6.x'
     - name: Install dependencies
       run: dotnet restore
     - name: Build