Is there any requirement in FCRA or ECOA that requires the Adverse Action notice be sent certified mail?

No.

That said, I am guessing you are asking because you currently send them via certified mail and are thinking about discontinuing this process. Before you discontinue it, make sure the certified mail wasn't the only way you were documenting that delivery of the AA notice. If it was, you will need to change your procedures so that you document when the notice was sent.