Bump NUnit from 4.2.0 to 4.2.1 in /src (#580)

Author: dependabot[bot]

src/Directory.Packages.props

@@ -6,7 +6,7 @@
   <ItemGroup>
     <PackageVersion Include="MarkdownSnippets.MsBuild" Version="27.0.2" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.11.0" />
-    <PackageVersion Include="NUnit" Version="4.2.0" />
+    <PackageVersion Include="NUnit" Version="4.2.1" />
     <PackageVersion Include="NUnit3TestAdapter" Version="4.6.0" />
     <PackageVersion Include="ProjectDefaults" Version="1.0.134" />
     <PackageVersion Include="SixLabors.ImageSharp" Version="3.1.5" />