chore: Update Senparc.CO2NET.WebApi version to 2.1.6-preview.1 and adjust Swashbuckle.AspNetCore.Annotations references for .NET 8.0 and netstandard2.1

Incremented the version of Senparc.CO2NET.WebApi to 2.1.6-preview.1. Updated the Swashbuckle.AspNetCore.Annotations package reference to version 10.0.0 for .NET 8.0 and maintained version 6.5.0 for netstandard2.1 to ensure compatibility.

Author: JeffreySu

src/Senparc.CO2NET.WebApi/Senparc.CO2NET.WebApi.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>netstandard2.1;net8.0</TargetFrameworks>
-    <Version>2.1.5</Version>
+    <Version>2.1.6-preview.1</Version>
     <LangVersion>latest</LangVersion>
     <AssemblyName>Senparc.CO2NET.WebApi</AssemblyName>
     <RootNamespace>Senparc.CO2NET.WebApi</RootNamespace>
@@ -51,7 +51,7 @@
 			[2025-08-20] v2.1.2 feat: Add default value GET to DefaultRequestMethod proprety
 			[2025-08-20] v2.1.2.1 Update async detection logic in DocMethodInfo and increment project version
 			[2025-09-06] v2.1.3 base module update
-			[2025-11-21] v2.1.5 feat: update Swashbuckle.AspNetCore to v10.0.0
+			[2025-11-21] v2.1.5 feat: update Swashbuckle.AspNetCore.Annotations to v10.0.0 for .NET 8.0 (netstandard2.1 remains on v6.5.0 for compatibility)
 </PackageReleaseNotes>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)' == 'Release' ">
@@ -70,10 +70,13 @@
   <ItemGroup>
     <None Include="..\Senparc.CO2NET\icon.jpg" Pack="true" Visible="false" PackagePath="" />
   </ItemGroup>
-  <ItemGroup>
+  <ItemGroup Condition="'$(TargetFramework)' == 'net8.0'">
     <PackageReference Include="Swashbuckle.AspNetCore.Annotations" Version="10.0.0" />
-    <!--<PackageReference Include="System.Reflection.Emit" Version="4.7.0" />-->
   </ItemGroup>
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.1'">
+    <PackageReference Include="Swashbuckle.AspNetCore.Annotations" Version="6.5.0" />
+  </ItemGroup>
+  <!--<PackageReference Include="System.Reflection.Emit" Version="4.7.0" />-->
   <ItemGroup Condition="'$(TargetFramework)' != 'netstandard2.1'">
     <PackageReference Include="Microsoft.AspNetCore.App" />
     <PackageReference Include="Microsoft.Extensions.DependencyModel" Version="8.0.0" />