revert to netstandard2.0 framework

Author: cg-zhou

StdEx/StdEx.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;net6.0;net8.0;net9.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0</TargetFrameworks>
     <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
     <PackageId>StdEx</PackageId>