Fix DisplayAlert to properly pass the cancel parameter to DisplayAlertAsync (#31474)

### Description of Change

Fixed ArgumentNullException in `Page.DisplayAlert(string title, string
message, string accept, string cancel, FlowDirection flowDirection)`.

### Issues Fixed

Fixes #31473

Author: jfversluis

src/Controls/src/Core/Page/Page.cs

@@ -350,7 +350,7 @@ public Task DisplayAlert(string title, string message, string cancel, FlowDirect
 		/// <inheritdoc cref="DisplayAlertAsync(string, string, string, string, FlowDirection)"/>
 		[Obsolete("Use DisplayAlertAsync instead")]
 		public Task<bool> DisplayAlert(string title, string message, string accept, string cancel, FlowDirection flowDirection)
-			=> DisplayAlertAsync(title, message, accept, null, flowDirection);
+			=> DisplayAlertAsync(title, message, accept, cancel, flowDirection);
 
 		/// <inheritdoc cref="DisplayAlertAsync(string, string, string, string, FlowDirection)"/>
 		public Task DisplayAlertAsync(string title, string message, string cancel)