[GREENFOOT-853] Google Maps API issue

The Google Maps API has updated such that you can't use it now without an API key (see https://cloud.google.com/maps-platform/user-guide/account-changes/ for some details). We should either document how to obtain an API key, or just remove the Map class entirely if it's not easy to get a free key for educational use.

Issue metadata

Issue type: Bug
Priority: Medium
Fix versions: 3.7.0