Update to netcoreapp2.1

Topten.JsonKit.Test/Topten.JsonKit.Test.csproj

@@ -3,7 +3,7 @@
   <Import Project="../buildtools/Topten.props" />
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <IsPackable>false</IsPackable>
   </PropertyGroup>
 

Topten.JsonKit/Topten.JsonKit.csproj

@@ -3,7 +3,7 @@
   <Import Project="../buildtools/Topten.props" />
 
   <PropertyGroup>
-    <TargetFrameworks>net46;netcoreapp2.0</TargetFrameworks>
+    <TargetFrameworks>net46;netcoreapp2.1</TargetFrameworks>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)'=='Release'">