Remove classes props from component

[joebochill]

Describe the desired feature/functionality

Remove the classes props from components.

Additional Context (where / how would this be used)

With the sx styles, it's much easier to target nested elements for styling, so this is unnecessary and bloats the components.

Is this request related to a current issue?

Suggested implementation details

Replace the classes prop with a hardcoded class name on various structural elements, all prefixed with Blui<ComponentName>-. Refactor styles to use sx and nested sx rules where appropriate.

[joebochill]

After looking at this, I don't think it'ss hurting anything to keep these classes in — it just give YET ANOTHER way to do style overriding (possibly adding confusion and inconsistency). So I don't think we really need to change this, but I just wanted to capture the idea in case we want to simplify things.

This would be a breaking change, so we could roll it in with the MUI 6 migration in v7 if we wanted to do it.