Bump Newtonsoft.Json in /samples/xamarin/UserItems/xamarin.forms/iOS

Bumps [Newtonsoft.Json](https://github.com/JamesNK/Newtonsoft.Json) from 9.0.1 to 13.0.1.
- [Release notes](https://github.com/JamesNK/Newtonsoft.Json/releases)
- [Commits](https://github.com/JamesNK/Newtonsoft.Json/compare/9.0.1...13.0.1)

---
updated-dependencies:
- dependency-name: Newtonsoft.Json
  dependency-type: direct:production
...

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samples/xamarin/UserItems/xamarin.forms/iOS/packages.config

@@ -4,7 +4,7 @@
   <package id="Microsoft.NETCore.Platforms" version="1.0.1" targetFramework="xamarinios10" />
   <package id="Microsoft.Win32.Primitives" version="4.0.1" targetFramework="xamarinios10" />
   <package id="NETStandard.Library" version="1.6.0" targetFramework="xamarinios10" />
-  <package id="Newtonsoft.Json" version="9.0.1" targetFramework="xamarinios10" />
+  <package id="Newtonsoft.Json" version="13.0.1" targetFramework="xamarinios10" />
   <package id="PCLCrypto" version="2.0.147" targetFramework="xamarinios10" />
   <package id="PInvoke.BCrypt" version="0.3.2" targetFramework="xamarinios10" />
   <package id="PInvoke.Kernel32" version="0.3.2" targetFramework="xamarinios10" />