Mention Mapbox as former sponsor

Author: joto

_includes/sponsors.html

@@ -2,3 +2,5 @@
     <div><a href="https://www.geofabrik.de/"><img width="40" height="40" src="{% link sponsors/geofabrik.svg %}" alt="Geofabrik" title="Geofabrik"/></a></div>
     <div><a href="https://www.cybertec-postgresql.com/"><img height="30" src="{% link sponsors/cybertec.svg %}" alt="Cybertec" title="Cybertec"/></a></div>
 </div>
+
+<p>We also thank our former sponsor <a href="https://www.mapbox.com/">Mapbox</a> for their support over many years.</p>