Update target framework to .NET 9.0

The target framework for the project `Edi.Captcha.Tests.csproj` has been updated from .NET 8.0 to .NET 9.0. This change is reflected by modifying the `<TargetFramework>` element from `net8.0` to `net9.0`.

Author: EdiWang

src/Edi.Captcha.Tests/Edi.Captcha.Tests.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFramework>net9.0</TargetFramework>
     <IsPackable>false</IsPackable>
   </PropertyGroup>
   <ItemGroup>