refs and cleanup

Author: SimonCropp

src/Directory.Packages.props

@@ -10,7 +10,7 @@
     <PackageVersion Include="NodaTime" Version="3.2.1" />
     <PackageVersion Include="NUnit" Version="4.3.2" />
     <PackageVersion Include="NUnit3TestAdapter" Version="5.0.0" />
-    <PackageVersion Include="Polyfill" Version="7.20.0" />
+    <PackageVersion Include="Polyfill" Version="7.21.0" />
     <PackageVersion Include="ProjectDefaults" Version="1.0.148" />
     <PackageVersion Include="System.Collections.Immutable" Version="9.0.2" />
     <PackageVersion Include="System.IO.Compression" Version="4.3.0" />