refs and cleanup

Author: SimonCropp

src/Directory.Packages.props

@@ -6,7 +6,7 @@
   <ItemGroup>
     <PackageVersion Include="DiffPlex" Version="1.9.0" />
     <PackageVersion Include="MarkdownSnippets.MsBuild" Version="28.0.0-beta.2" />
-    <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.0.0" />
     <PackageVersion Include="NUnit" Version="4.4.0" />
     <PackageVersion Include="NUnit3TestAdapter" Version="5.1.0" />
     <PackageVersion Include="Polyfill" Version="8.9.0" />