switch ref to network for orange roads route colouring

[nvkelso]

There should be no visual change, but as properties are stripped out at low zooms, we should use the reliable Tilezen ones instead of the random OSM ones per https://github.com/tilezen/vector-datasource/pull/2008 and https://github.com/tilezen/vector-datasource/issues/1998.

I switch this to network instead of shield_text because at some zooms shield_text is dropped to reduce tile size and increase feature merging.

SF: 12/37.7582/-122.2903

before

after

London: 11/51.4809/-0.0778

before

after