New OpenStreetMap Carto release, v4.21.0

[pnorman]

A new version of OpenStreetMap Carto, v4.21.0, has been released. There are a few deployment-related changes

• world_boundaries-spherical.tgz can be removed

• The URLs for the water and icesheet shapefiles have changed from openstreetmapdata.com to osmdata.openstreetmap.de

• The military area index has adjusted. The old one can be dropped and then the new one created concurrently with

DROP INDEX planet_osm_polygon_military;
CREATE INDEX CONCURRENTLY planet_osm_polygon_military
  ON planet_osm_polygon USING GIST (way)
  WHERE (landuse = 'military' OR military = 'danger_area') AND building IS NULL;

[pnorman]

A potential bug has been raised with v4.21.0, I recommend holding off for now until we see if a v4.21.1 is needed

[HolgerJeromin]

Ref gravitystorm/openstreetmap-carto#3736 (comment)

[pnorman]

Closed for #247