[ios] fix memory leak in Entry

[jonathanpeppers]

Context: #16346

This addresses the memory leak discovered by:

src/Core/src/Platform/iOS/MauiTextField.cs(69,32): error MA0001: Event 'SelectionChanged' could cause memory leaks in an NSObject subclass. Remove the event or add the [UnconditionalSuppressMessage("Memory", "MA0001")] attribute with a justification as to why the event will not leak.
src/Core/src/Platform/iOS/MauiTextField.cs(68,30): error MA0001: Event 'TextPropertySet' could cause memory leaks in an NSObject subclass. Remove the event or add the [UnconditionalSuppressMessage("Memory", "MA0001")] attribute with a justification as to why the event will not leak.

I could reproduce a leak in a test such as:

await InvokeOnMainThreadAsync(() =>
{
    var layout = new Grid();
    var entry = new Entry();
    layout.Add(entry);
    var handler = CreateHandler<LayoutHandler>(layout);
    viewReference = new WeakReference(entry);
    handlerReference = new WeakReference(entry.Handler);
    platformViewReference = new WeakReference(entry.Handler.PlatformView);
});

await AssertionExtensions.WaitForGC(viewReference, handlerReference, platformViewReference);
Assert.False(viewReference.IsAlive, "Entry should not be alive!");
Assert.False(handlerReference.IsAlive, "Handler should not be alive!");
Assert.False(platformViewReference.IsAlive, "PlatformView should not be alive!");