update Newtonsoft.Json to 13.0.3 (#174)

Author: collinstevens

Source/AsyncGenerator.Tests/AsyncGenerator.Tests.csproj

@@ -27,6 +27,7 @@
     <PackageReference Include="Microsoft.Extensions.Logging.Console" Version="6.0.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.2.0" />
     <PackageReference Include="Microsoft.Build.Locator" Version="$(MicrosoftBuildLocatorVersion)" />
+    <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
     <PackageReference Include="NUnit" Version="3.14.0" />
     <PackageReference Include="NUnit.Analyzers" Version="4.0.1">
       <PrivateAssets>all</PrivateAssets>