Fix incorrect BasedOn guidance for WinUI 3 controls in xaml-styles.md

[Copilot]

The "Avoid restyling controls" section incorrectly told WinUI 3 developers to omit BasedOn for Microsoft.UI.Xaml.Controls controls. In WinUI 3, omitting BasedOn completely replaces the default style—stripping rounded corners, theming, and all other WinUI 3 visual defaults (as the reporter's screenshots show).

Changes

• Correct the BasedOn guidance: Replace the two-namespace paragraph split (which conflated WinUI 2/UWP with WinUI 3) with a single unambiguous rule: always use BasedOn="{StaticResource Default<ControlName>Style}" for Microsoft.UI.Xaml.Controls controls in WinUI 3.
• Remove off-topic UWP/WinUI 2 paragraph: The Windows.UI.Xaml.Controls guidance was irrelevant to WinUI 3 apps and caused the namespace confusion the reporter hit.
• Update code example to use Button/DefaultButtonStyle matching the reporter's scenario.
• Update ms.date to reflect the substantive content change.

<!-- Correct: inherits WinUI 3 defaults (rounded corners, theming), overrides only Foreground -->
<Style TargetType="Button" BasedOn="{StaticResource DefaultButtonStyle}">
    <Setter Property="Foreground" Value="Blue"/>
</Style>

<!-- Wrong (old guidance): replaces the entire default style, loses rounded corners -->
<Style TargetType="Button">
    <Setter Property="Foreground" Value="Blue"/>
</Style>

• Fixes restyling controls on WinUI3 #5098

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Learn Build status updates of commit 00889e7:

✅ Validation status: passed

File	Status	Preview URL	Details
hub/apps/develop/platform/xaml/xaml-styles.md	✅Succeeded

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