Context: dotnet#14664
Context: https://github.com/nacompllo/MemoryLeakEverywhere/tree/bugfix/memoryLeakItemsSource

In reviewing our latest changes in main with the above customer sample,
I found that there appeared to be leaks related to MAUI's
`UITableViewCell` subclasses when using `CollectionView`.

I was able to reproduce the issue in a test, such as:

    // DataTemplate saves WeakReference to the View in a list
    collectionView.ItemTemplate = new DataTemplate(() =>
    {
        var label = new Label();
        labels.Add(new(label));
        return label;
    });

    // Create a cell and bind it to the template:
    cell = new VerticalCell(CGRect.Empty);
    cell.Bind(collectionView.ItemTemplate, bindingContext, collectionView);

    // Check we have no leaks
    foreach (var reference in labels)
    {
        Assert.False(reference.IsAlive, "View should not be alive!");
    }

After isolating the issue, I found the issue was the
`TemplatedCell.PlatformHandler` property:

    internal IPlatformViewHandler PlatformHandler { get; private set; }

This stores a copy of the `LabelHandler` in our test/example.

The problem with `UITableViewCell` is that UIKit holds onto these and
reuses them. This means that UIKit may keep the `LabelHandler` alive
longer than needed.

It also appears to be a somewhat complex circular reference:

* `CollectionView` -> handlers / etc. -> `TemplatedCell` -> `LabelHandler` -> `Label` -> `CollectionView`

I made the `PlatformHandler` use a `WeakReference` as its backing
field and the problem goes away!

I will retest dotnet#14664 to verify if it is fully solved.