There is no such thing as MQTT Persistence in OH 3. It was only ever supported in the 1.x version of the binding, and even then it was only barely supported since the addition of the MQTT Event Bus feature.
mqtt.cfg, mqtt-eventbus.cfg, and mqtt-persistence.cfg are no longer used. You’ll have to look at the MQTT add-on docs for how to create an MQTT Broker Thing with the connection info and then follow the link below for how to set up the publishing part of the MQTT EventBus post.