I had my battery replaced and now I have an alert that it is an unknown part. How can I get rid of this

As mentioned…if it wasn’t repaired by Apple or an authorized repair facility with an authentic Apple battery you can not get rid of the message. Apple is very explicit that all repairs should be done by them or an authorized facility with exception to any repairs they allow you to do as a self repair with authorized parts and tools.

Apple Repair and Repair Status Check - Official Apple Support

Self Service Repair - Apple Support

If Apple or an authorized repair center did not do the replacement, you will never get rid of that message.