bitcoin/bitcoin#31130 dropped UPnP support. It's now recommended that
users use PCP (with NATPMP fallback).

But this is not explained in a friendly manner:

A user would have to figure out that they need to manually edit settings.json or delete it and redo all their settings.

We should probably automatically delete it from settings.json. And then display a warning / info (instead of error) that tells users to consider the PCP setting in the GUI.