What is the equivalent of a buck converter for AC voltage?
A transformer would probably be the most efficient. An electronic dimmer would probably be a close second.

A transformer would probably be less expensive, but finding one for the right voltage may be difficult. Transformers can be wired to "buck" the voltage. To do that, you connect the primary in series with the secondary. That creates an autotransformer, so there is no isolation.

With a dimmer, all you need to do id dial back the voltage.

With either, you need to figure out what to do about an enclosure. You can buy dimmers designed to mount in a wall box. Getting a suitable enclosed transformer may be more difficult. It is unlikely that any solution will conform to electrical codes.