BindClass method's anchor should be nullable

[stevemonaco]

Describe the bug

Continuing from #15711 (reply in thread)

Current signature is:

public static class StyledElementExtensions
{
	public static IDisposable BindClass(this StyledElement target, string className, IBinding source, object anchor) =>
		ClassBindingManager.Bind(target, className, source, anchor);
}

anchor doesn't seem to be used anymore after the 11.1 bindings refactor. So I propose:

• Changing StyledElementExtensions.BindClass to object? anchor
• Changing ClassBindingManager.Bind's anchor parameter to object? anchor (or removing it, since it's internal)

Alternative:

• Create new overload of StyledElementExtensions.BindClass without anchor. Mark other overload as obsolete to be removed in v12.
• Changing ClassBindingManager.Bind's anchor parameter to object? anchor (or removing it, since it's internal)

To Reproduce

CS8625: Cannot convert null literal to non-nullable reference type

var text = new TextBlock();

var mb = new MultiBinding()
{
    Bindings = [new Binding("IsA"), new Binding("IsB")],
    Converter = BoolConverters.And
};

text.BindClass("big", mb, null);

Expected behavior

No nullable warning. No need to use null!.

Avalonia version

Current master

OS

No response

Additional context

No response