remove openmapsurfer tile layer

[tyrasd]

The korona.geog.uni-heidelberg.de server was shut down. OpenMapSurfer tiles are now available from a new URL. Normally, one would need an API key for this, but for umap you can use https://maps.heigit.org/openmapsurfer/tiles/roads/webmercator/{z}/{x}/{y}.png for now (//cc @TimMcCauley). See https://ask.openrouteservice.org/t/openmapsurfer-api/233 for more information.

[joker234]

The mapsurfer tiles are not available anymore and should be removed permanently. See also:

• remove OpenMapSurfer tile sources leaflet-extras/leaflet-providers#354 (comment)
• https://ask.openrouteservice.org/t/mapsurfer-discontinued/1873
• uMap: OSM Roads not available anymore osm-fr/infrastructure#319

[tyrasd]

This was solved on the "main" umap instance via osm-fr/infrastructure#319