fix: src/Auth0.Core/Auth0.Core.csproj to reduce vulnerabilities (#560)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-DOTNET-NEWTONSOFTJSON-2774678

Co-authored-by: snyk-bot <snyk-bot@snyk.io>
Co-authored-by: Evan Sims <evan.sims@auth0.com>

src/Auth0.Core/Auth0.Core.csproj

@@ -10,7 +10,7 @@
     <AssemblyOriginatorKeyFile>..\..\build\Auth0NetStrongName.snk</AssemblyOriginatorKeyFile>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Newtonsoft.Json" Version="12.0.3" />
+    <PackageReference Include="Newtonsoft.Json" Version="13.0.1" />
   </ItemGroup>
   <ItemGroup>
     <Folder Include="Properties\" />