Revert "Add netstandard2.0 to SpecFileGen"

This reverts commit f9aa8e1.

Author: tats-u

src/SpecFileGen/SpecFileGen.csproj

@@ -2,8 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFrameworks>netstandard2.0;net6.0;net8.0;net10.0</TargetFrameworks>
-    <LangVersion>latest</LangVersion>
+    <TargetFrameworks>net6.0;net8.0;net10.0</TargetFrameworks>
     <ImplicitUsings>enable</ImplicitUsings>
     <IsPackable>false</IsPackable>
     <NoWarn>$(NoWarn);NETSDK1138</NoWarn>