The core snaps are actually the base of the sandbox a snap runs in, an application snap declares which base it is built against and snapd will make sure the related base gets installed alongside with the app…
The snapd snap itself always uses the latest base (core24 currently)
You seem to have certbot installed, my guess would be that certbot uses/requires the core20 base (you can check with snap info --verbose certbot) … if this is not the case, you can indeed safely remove the core20 snap