Update to .NET 9.0: global.json and test project

Co-authored-by: BenjaminMichaelis <[email protected]>

Author: Copilot

IntelliTect.Multitool.Tests/IntelliTect.Multitool.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 	<Import Project="../IntelliTect.Multitool/Build/IntelliTect.Multitool.targets" />
   <PropertyGroup>
-    <TargetFrameworks>net8.0</TargetFrameworks>
+    <TargetFrameworks>net9.0</TargetFrameworks>
 
     <IsPackable>false</IsPackable>
   </PropertyGroup>

global.json

@@ -1,6 +1,6 @@
 {
   "sdk": {
-    "version": "8.0.100",
+    "version": "9.0.100",
     "rollForward": "minor",
     "allowPrerelease": false
   }