Updated .net framework used for testing.

Author: neogeek

RhythmGameUtilities.Tests/RhythmGameUtilities.Tests.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
     <PropertyGroup>
-        <TargetFramework>net8.0</TargetFramework>
+        <TargetFramework>net9.0</TargetFramework>
         <LangVersion>9</LangVersion>
         <IsPackable>false</IsPackable>
         <RootNamespace>RhythmGameUtilities.Tests</RootNamespace>