Use dotnet 9 for unit tests project

Author: denis-ivanov

src/AngleSharp.XPath.Tests/AngleSharp.XPath.Tests.csproj

@@ -1,4 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="AngleSharp.Xml" Version="1.0.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.13.0" />
@@ -9,8 +13,4 @@
   <ItemGroup>
     <ProjectReference Include="..\AngleSharp.XPath\AngleSharp.XPath.csproj" />
   </ItemGroup>
-  <PropertyGroup>
-    <TargetFramework>net6.0</TargetFramework>
-    <IsPackable>false</IsPackable>
-  </PropertyGroup>
 </Project>