Revert back to targeting netcoreapp3.1

NET 6 single exe publishing doesn't work on Windows 7

Author: RayCarrot

src/RayCarrot.Ray1Editor/RayCarrot.Ray1Editor.csproj

@@ -1,16 +1,15 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">
 
   <!-- General -->
   <PropertyGroup>
     <OutputType>WinExe</OutputType>
-    <TargetFramework>net6.0-windows</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <UseWPF>true</UseWPF>
     <LangVersion>10</LangVersion>
     <ApplicationIcon>UI\Img\AppIcon.ico</ApplicationIcon>
     <Version>0.2.0.0</Version>
     <Authors>RayCarrot</Authors>
     <Product>Ray1Editor</Product>
-    <IncludeNativeLibrariesForSelfExtract>true</IncludeNativeLibrariesForSelfExtract>
     <PackageProjectUrl>https://github.com/RayCarrot/RayCarrot.Ray1Editor</PackageProjectUrl>
     <RepositoryUrl>https://github.com/RayCarrot/RayCarrot.Ray1Editor</RepositoryUrl>
   </PropertyGroup>