Adjust min_zoom of OSM highways and major roads to better match Natural Earth

[nvkelso]

@sensescape is adding optional level of detail filters to Refill and is making use of the min_zoom to do the filtering. But for the roads layer there's a discontinuity between the Natural Earth data at zooms 0-7 and the OSM data that starts at 8. Many roads that already showed up in earlier zooms only have a min_zoom of 8 set, when they should have an earlier zoom.

• https://github.com/tilezen/vector-datasource/blob/master/yaml/roads.yaml#L185-L204

  #############################################################
  #
  # OSM highway
  #
  #############################################################
  - filter:
      highway: motorway
    # min_zoom 8 ≤ any mz_calculate_path_major_route(), so skip
    min_zoom: 8
    table: osm
    output:
      <<: *osm_highway_properties
      <<: *osm_network
      kind: highway
      kind_detail: motorway
  - filter:
      highway: [trunk, primary]
    # min_zoom 8 ≤ any mz_calculate_path_major_route(), so skip
    min_zoom: 8
    table: osm
    output:
      <<: *osm_highway_properties
      <<: *osm_network
      kind: major_road
      kind_detail: {col: highway}

[nvkelso]

One solution:

• highway: motorway at min_zoom: 5
• highway: trunk at min_zoom: 6
• highway: primary at min_zoom: 8

[nvkelso]

Related: #1280

[nvkelso]

Subtask of #1349.

[nvkelso]

Verified: