Update test project to net9

Author: flsl0

src/Fli.Tests/Fli.Tests.fsproj

@@ -2,7 +2,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <OutputType>Library</OutputType>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFramework>net9.0</TargetFramework>
     <IsPackable>false</IsPackable>
   </PropertyGroup>
   <ItemGroup>
@@ -25,4 +25,4 @@
     <ProjectReference Include="..\Fli\Fli.fsproj" />
   </ItemGroup>
   <Import Project="..\..\.paket\Paket.Restore.targets" />
-</Project>
+</Project>