Update vehicle counts layer to show all years and emphasize data source

[dabreegster]

Just the first task from #482. Before pushing, we need to update the pmtiles file

[dabreegster]

S3 updated now, you can generate yourself or copy https://atip.uk/layers/v1/vehicle_counts.pmtiles to GCS. Please update manifests when done, and I'll merge then, so there's minimal outage time