Only target .NET Standard 2.0 (netstandard2.0)

See #20 (comment)

Author: generalmimon

kaitai_struct_runtime_csharp.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net6.0;net462</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0</TargetFrameworks>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <NoWarn>1701;1702;CS1591</NoWarn>