Context: dotnet#16346

This addresses the memory leak discovered by:

    src/Core/src/Platform/iOS/MauiTextView.cs(37,30): error MA0001: Event 'TextSetOrChanged' 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/MauiTextView.cs(12,20): error MA0002: Member '_placeholderLabel' could cause memory leaks in an NSObject subclass. Remove the member, store the value as a WeakReference, or add the [UnconditionalSuppressMessage("Memory", "MA0002")] attribute with a justification as to why the member will not leak.

I could reproduce a leak in a test such as:

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

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

I will create a similar PR for `Entry` as well.