Bring zoom references in line with coding standard

Replace [zoom=13] with [zoom >= 13][zoom < 14]
Merge branch 'zoom' of https://github.com/math1985/openstreetmap-carto into math1985-zoom
Remove the unnecessary < 14 restrictions.

The symbolizers and properties created at z13 are used at z14 so
don't need to be restricted.
Merge branch 'math1985-zoom'