Use the recommended OSM Tile Server URL scheme

> TL;DR: Please use https://tile.openstreetmap.org/{z}/{x}/{y}.png - no subdomains and no short names. For more details, see the related [issue][issue]

Historically, the use of subdomains (i.e. `a.tile.openstreetmap.org`) or shortened domain names for the tile server URLs (i.e. `b.tile.osm.org`) has been a standard practice.

However, since the modernised OpenStreetMap Tile server supports HTTP/2 and HTTP/3, the old a|b|c aliases are no longer needed to increase browser connection concurrency.

Besides, using single hostname improves cache efficiency and reduces operational complexity. That's why, the new preferred URL scheme for the OSM Tiles is now `https://tile.openstreetmap.org/{z}/{x}/{y}.png`. The old URLs add maintenance burden and may stop working in the future.

I made the relevant changes, and created a PR for this issue.

[issue]: https://github.com/openstreetmap/operations/issues/737

Search the repository for tile.openstreetmap.org, tile.osm.org, and a/b/c tile-server URL forms. Update the existing tile references to use the single recommended URL scheme, then verify that no old subdomain or shortened-domain forms remain.

Written by the indexing model from the issue text.