feat: change target framework to lower version

Author: insthync

SimpleNetworkManager.NET/SimpleNetworkManager.NET.csproj

@@ -1,7 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
+    <LangVersion>8.0</LangVersion>
+    <TargetFramework>netstandard2.1</TargetFramework>
     <ImplicitUsings>disable</ImplicitUsings>
     <Nullable>enable</Nullable>
     <RootNamespace>Insthync.SimpleNetworkManager.NET</RootNamespace>