Update system.text.json to the latest (#671)

test/Microsoft.NET.Sdk.Functions.Generator.Tests/Microsoft.NET.Sdk.Functions.Generator.Tests.csproj

@@ -17,6 +17,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.8.0" />
     <PackageReference Include="xunit" Version="2.3.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.3.1" />
+    <PackageReference Include="System.Text.Json" Version="6.0.11" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\src\Microsoft.NET.Sdk.Functions.Generator\Microsoft.NET.Sdk.Functions.Generator.csproj" />