Update target framework to .NET 9.0

The project file `Edi.Captcha.SampleApp.csproj` has been modified to update the target framework from .NET 8.0 to .NET 9.0. This change is made by replacing `<TargetFramework>net8.0</TargetFramework>` with `<TargetFramework>net9.0</TargetFramework>`.

Author: EdiWang

src/Edi.Captcha.SampleApp/Edi.Captcha.SampleApp.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk.Web">
 
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFramework>net9.0</TargetFramework>
   </PropertyGroup>
 
   <ItemGroup>