migrate LogicalKeySet to SingleActivator in shorcuts

[yellowgraysu]

Related to https://github.com/flutter/flutter/pull/80756

I want to remove some key in shortcuts. The following code does not work because the type of shortcuts in VRouter is Map<LogicalKeySet, Intent> but the type of
WidgetsApp.defaultShortcuts is Map<ShortcutActivator, Intent>

VRouter(
    shortcuts: Map.from(WidgetsApp.defaultShortcuts)..remove(
        // some key
    ),
    //...
),

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