refs and cleanup

Author: SimonCropp

src/Directory.Packages.props

@@ -8,7 +8,7 @@
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
     <PackageVersion Include="NUnit" Version="4.1.0" />
     <PackageVersion Include="NUnit3TestAdapter" Version="4.5.0" />
-    <PackageVersion Include="Polyfill" Version="5.6.0" />
+    <PackageVersion Include="Polyfill" Version="6.0.0" />
     <PackageVersion Include="ProjectDefaults" Version="1.0.131" />
     <PackageVersion Include="System.Collections.Immutable" Version="8.0.0" />
     <PackageVersion Include="System.Memory" Version="4.5.5" />