Bump Newtonsoft.Json in /test/WorkflowCore.TestAssets

Bumps [Newtonsoft.Json](https://github.com/JamesNK/Newtonsoft.Json) from 12.0.1 to 13.0.1.
- [Release notes](https://github.com/JamesNK/Newtonsoft.Json/releases)
- [Commits](JamesNK/Newtonsoft.Json@12.0.1...13.0.1)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

test/WorkflowCore.TestAssets/WorkflowCore.TestAssets.csproj

@@ -40,7 +40,7 @@
   <ItemGroup>
     <PackageReference Include="FluentAssertions" Version="4.19.4" />
     <PackageReference Include="Moq" Version="4.7.145" />
-    <PackageReference Include="Newtonsoft.Json" Version="12.0.1" />
+    <PackageReference Include="Newtonsoft.Json" Version="13.0.1" />
     <PackageReference Include="NUnit" Version="3.6.1" />
   </ItemGroup>