fix: invalid winui usings on uwp build

[Xiaoy312]

GitHub Issue (If applicable): n/a

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

• invalid winui usings are used for uwp-desktop head build following the read-me instructions
• DependencyObject::Dispatcher is not implemented/supported/defined on winui, as a result attempting to access its member is an automatic NullReferenceException on winui.

What is the new behavior?

^ fixed

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] Tested code with current supported SDKs
• [x] Docs have been added/updated which fit documentation template. (for bug fixes / features)
• [ ] Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• [ ] Wasm UI Tests are not showing unexpected any differences. Validate PR Screenshots Compare Test Run results.
• [x] Contains NO breaking changes
• [ ] Updated the Release Notes
• [ ] Associated with an issue (GitHub or internal)

[Xiaoy312]

@jeromelaban btw, is it intentional that even "uno core team" dont have write-access to this repo?

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