[windows] fix memory leak when CollectionView.ItemsSource changes

Fixes: #13393

Context: https://github.com/ivan-todorov-progress/maui-collection-view-memory-leak-bug

Debugging the above sample, I found that `ItemsView._logicalChildren`
grew indefinitely in size if you replace a `CollectionView`'s
`ItemsSource` over and over.

I could fix this by creating a new `internal`
`ItemsView.ClearLogicalChildren()` method and call it from the Windows
`ItemsViewHandler`.

WIP to determine if we can write a test or if this happens on any
other platforms. Android seems OK.

src/Controls/src/Core/Handlers/Items/ItemsViewHandler.Windows.cs

@@ -123,6 +123,7 @@ protected virtual void CleanUpCollectionViewSource()
 				CollectionViewSource.Source = null;
 				CollectionViewSource = null;
 			}
+			VirtualView?.ClearLogicalChildren();
 
 			if (VirtualView?.ItemsSource == null)
 			{

src/Controls/src/Core/Items/ItemsView.cs

@@ -142,6 +142,15 @@ public void RemoveLogicalChild(Element element)
 			VisualDiagnostics.OnChildRemoved(this, element, oldLogicalIndex);
 		}
 
+		internal void ClearLogicalChildren()
+		{
+			// Reverse for-loop, so children can be removed while iterating
+			for (int i = _logicalChildren.Count - 1; i >= 0; i--)
+			{
+				RemoveLogicalChild(_logicalChildren[i]);
+			}
+		}
+
 		internal override IReadOnlyList<Element> LogicalChildrenInternal => _logicalChildren.AsReadOnly();
 
 		internal static readonly BindableProperty InternalItemsLayoutProperty =