[tests] On 9.0.4 theres no build warning for System.Text.Json

[rmarinho]

Description of Change

Buy moving to 9.0.4 the AOT tests for root all assemblies was failing since theres no build warning for System.Text.Json anymore.

Failing build
https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=11478717&view=ms.vss-test-web.build-test-results-tab&runId=128335150&resultId=100004&paneView=debug

[Copilot]

Pull Request Overview

This PR removes the expected build warning for System.Text.Json in the AOT tests for root all assemblies, addressing failures seen after the 9.0.4 update.

• Removed the hard-coded expected warning message configured for System.Text.Json
• Simplified the expectedNativeAOTWarnings list to an empty list

Comments suppressed due to low confidence (1)

src/TestUtils/src/Microsoft.Maui.IntegrationTests/Utilities/BuildWarningsUtilities.cs:129

• [nitpick] Consider adding a comment on this line explaining that the expected warning for System.Text.Json has been removed due to the 9.0.4 update, to clarify the change in behavior for future maintainers.

private static readonly List<WarningsPerFile> expectedNativeAOTWarnings = new();