Update sample code for .NET 6?

[wjk]

What the title says.

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Document Details

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• ID: fd478913-8219-1440-f2c7-a227238f1607
• Version Independent ID: 78ab449b-deda-f194-cd61-2219b2ac3c23
• Content: Using the Visual Layer with Windows Forms - Windows apps
• Content Source: hub/apps/desktop/modernize/using-the-visual-layer-with-windows-forms.md
• Product: uwp
• Technology: apps
• GitHub Login: @jwmsft
• Microsoft Alias: jimwalk