Revert Directory.Build.props to original state - keep .NET 9 and C# 13.0 as originally configured

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

Author: Copilot

src/.build-system/Directory.Build.props

@@ -2,8 +2,8 @@
 	<Import Project="./BuildVariables.props" />
 	<!-- All C# Projects -->
 	<PropertyGroup Label="C# Project Property Groups" Condition="'$(IsCSharpProject)' == 'true'">
-		<TargetFramework>net8.0</TargetFramework>
-		<LangVersion>12.0</LangVersion>
+		<TargetFramework>net9.0</TargetFramework>
+		<LangVersion>13.0</LangVersion>
 		<ImplicitUsings>enable</ImplicitUsings>
 		<Nullable>enable</Nullable>
 		<AccelerateBuildsInVisualStudio>true</AccelerateBuildsInVisualStudio>