Update Directory.Packages.props

Author: SimonCropp

src/Directory.Packages.props

@@ -8,7 +8,7 @@
     <PackageVersion Include="CommandLineParser" Version="2.9.1" />
     <PackageVersion Include="Microsoft.Build.Tasks.Core" Version="17.14.8" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.0.0" />
-    <PackageVersion Include="Polyfill" Version="9.0.0-beta.9" />
+    <PackageVersion Include="Polyfill" Version="9.0.0-beta.19" />
     <PackageVersion Include="ProjectDefaults" Version="1.0.161" />
     <PackageVersion Include="System.Memory" Version="4.6.3" />
     <PackageVersion Include="System.Net.Http" Version="4.3.4" />