Replace Nullable with PolySharp as a dependency

PolySharp is far more exhaustive in the polyfills than Nullable is.

Author: AArnott

Directory.Packages.props

@@ -15,7 +15,7 @@
     <GlobalPackageReference Include="DotNetAnalyzers.DocumentationAnalyzers" Version="1.0.0-beta.59" />
     <!-- The condition works around https://github.com/dotnet/sdk/issues/44951 -->
     <GlobalPackageReference Include="Nerdbank.GitVersioning" Version="3.6.146" Condition="!('$(TF_BUILD)'=='true' and '$(dotnetformat)'=='true')" />
-    <GlobalPackageReference Include="Nullable" Version="1.3.1" />
+    <GlobalPackageReference Include="PolySharp" Version="1.15.0" />
     <GlobalPackageReference Include="StyleCop.Analyzers.Unstable" Version="1.2.0.556" />
   </ItemGroup>
 </Project>