Context: dotnet#16346

This addresses the memory leak discovered by:

    src/Core/src/Platform/iOS/MauiSearchBar.cs(36,63): 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/MauiSearchBar.cs(54,32): error MA0001: Event 'OnMovedToWindow' 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/MauiSearchBar.cs(55,32): error MA0001: Event 'EditingChanged' 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/MauiSearchBar.cs(70,31): error MA0003: Subscribing to events with instance method 'OnEditingChanged' could cause memory leaks in an NSObject subclass. Remove the subscription or convert the method to a static method.

I could reproduce a leak in a test such as:

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

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

Solved the issues by making a non-NSObject `MauiSearchBarProxy` class.

I found & fixed a couple related issues:

* `SearchBarExtensions.GetSearchTextField()` would have always thrown
  `StackOverlowException` if iOS was less than 13? It just called itself?!?

* Removed `MauiSearchBar.EditingChanged`, as we could subscribe to the
  event from the `UITextField` directly instead.