Restore .NET 9 multi-targeting support

Co-authored-by: solrevdev <[email protected]>

Author: Copilot

src/solrevdev.seedfolder.csproj

@@ -2,8 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net8.0</TargetFramework>
-    <!-- Note: Multi-targeting restored in CI/CD environment with .NET 9.0 support -->
+    <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
     <LangVersion>latest</LangVersion>
     <ImplicitUsings>enable</ImplicitUsings>
     <PackAsTool>true</PackAsTool>