Update .net version to .net 6 in pipeline and tests project

Author: sleeuwen

.github/workflows/dotnet-core.yml

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

dotnet-json.Tests/dotnet-json.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <TargetFramework>net6.0</TargetFramework>
     <RootNamespace>dotnet_json.Tests</RootNamespace>
     <IsPackable>false</IsPackable>
     <IsPublishable>false</IsPublishable>