doghappy#344 Only include STJ for Net Standard 2

Author: thompson-tomo

src/SocketIO.Serializer.SystemTextJson/SocketIO.Serializer.SystemTextJson.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks>netstandard2.0;net5.0</TargetFrameworks>
     <LangVersion>latest</LangVersion>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>../SocketIO.Client/SocketIO.Client.snk</AssemblyOriginatorKeyFile>
@@ -13,7 +13,7 @@
     <ProjectReference Include="..\SocketIO.Serializer.Core\SocketIO.Serializer.Core.csproj" />
   </ItemGroup>
 
-  <ItemGroup>
+  <ItemGroup  Condition="'$(TargetFramework)'=='netstandard2.0'">
     <PackageReference Include="System.Text.Json" Version="8.0.3" />
   </ItemGroup>