Complete obsoletion process for DomainUpDownAccessibleObject class.

[Tanya-Solyanik]

We'll be conservative in NET7 and will return instance of an obsoleted DomainUpDownAccessibleObject type from DomainUpDown.CreateAccessibilityInstance method. This will prevent failures of the runtime casts in snippets like this one:

internal class MySpinner : DomainUpDown
{
<snip>
DomainUpDownAccessibleObject ao = CreateAccessibleObject() as DomainUpDownAccessibleObject;
<snip>
}

In .NET 8 we will remove protected override methos DomainUpDown.CreateAccessibilityInstance. This kind of change is allowed by the breaking changes rules:

Introducing or removing an override

Make note, that introducing an override might cause previous consumers to skip over the override when calling base.

Additionally, we should consider promoting build time warning to an error because this class had always been documented as internal use only. Lets discuss this part of the change within the team.

Originally posted by @Tanya-Solyanik in https://github.com/dotnet/winforms/pull/7330#discussion_r944772133