Hello. Can anyone explain why I am seeing a pop-up message saying that the mo21.biz certificate has expired and how to avoid it? How can I remove this from my iPhone?

Check for and remove any unrecognized subscribed calendars, using the following procedure.

Delete spam calendars and events on iPhone - Apple Support

If you find a matching subscribed calendar, remove it and the certificate warning will be gone.

If you do not see a matching calendar subscription, let us know.

If a site's certificate has expired, that is something they have to fix on their end. You cannot fix it.