Deprecation of Singleton Manifests

[Trenly]

The singleton format has been deprecated in documentation only for more than a year. Since the time of its deprecation, all singleton manifests had, at one point or another, been scrubbed from the repository and updated to the multi-manifest format.

Moving forward, the deprecation will be strongly enforced, starting with a policy which simply warns about the deprecation. All users are encouraged to use tools like the Windows Package Manager Manifest Creator (a.k.a. WinGet-Create), YamlCreate.ps1, or Komac to create manifests compliant with the latest Manifest Specification