remove target framework netcoreapp3.1

Author: smdn

src/Smdn.Reflection.ReverseGenerating.ListApi.Core/Smdn.Reflection.ReverseGenerating.ListApi.Core.csproj

@@ -4,7 +4,7 @@ SPDX-License-Identifier: MIT
 -->
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>net8.0;net6.0;netcoreapp3.1</TargetFrameworks>
+    <TargetFrameworks>net8.0;net6.0</TargetFrameworks>
   </PropertyGroup>
 
   <Import Project="Smdn.Reflection.ReverseGenerating.ListApi.Core.Common.props" />