fix!: Remove OnWindowCreated from WinUI tree

[MartinZikmund]

BREAKING CHANGE: Application.OnWindowCreated does not exist in WinAppSDK

GitHub Issue (If applicable): part of #8339

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Internal Issue (If applicable):

[mergify]

⚠️ The sha of the head commit of this PR conflicts with #12594. Mergify cannot evaluate rules on this PR. ⚠️