This pull request includes several changes to the UWP project, focusing on updating package references for Newtonsoft.Json Fixes #9102, modifying project configurations, updating documentation, removing secure file download steps, modifying project files to use new paths for temporary test signing keys.

Key Changes:

Pipeline Configuration:

• Removed secure file download steps from .pipelines/templates/uwp-download-secure-files.yml and updated .pipelines/uwp-ci.yml to remove the template reference and add a task to generate a test signing certificate. [1] [2] [3]

NuGet Configuration:

• Updated NuGet package source in source/NuGet.config from AdaptiveCards-public to AdaptiveCards-Dependencies. New feed is publicly accessible allowing everyone to build this project.
• Removed local NuGet.config from UWP project as a single root NuGet.config is now used.

Project Configuration:

• Updated paths for key files in multiple project files (AdaptiveCardTestApp.csproj, PerfApp.vcxproj, UWPUITestApp.csproj, etc.) to use the Assets directory. [1] [2] [3] [4] [5] [6]
• Updated Newtonsoft.Json package references to version 13.0.3 in several project files. [1] [2] [3] [4] [5] [6]

Documentation:

• Updated documentation in source/uwp/README.md to reflect changes in the NuGet package feed from AdaptiveCards-public to AdaptiveCards-Dependencies. [1] [2] [3] [4] [5]