[.NET] Update version of Microsoft.Extension.Ai & System.Text.Json

[DavidLuong98]

Why are these changes needed?

#4031

• Update package
  • Microsoft.Extension.Ai to 9.0.0-preview.9.24525.1
  • System.Text.Json to 8.0.5
• Add NoWarnDuplicatePackages to Directory.Build.props and add the noWarn the necessary csproj. This is so the build does not warn about duplicate package reference

Related issue number

Closes #4031

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.