Comply with the latest flutter_map/OSM requirements

[monsieurtanuki]

We currently use flutter_map and OpenStreetMap tiles.
Maps may not be displayed in a near future.
https://operations.osmfoundation.org/policies/tiles/
https://docs.fleaflet.dev/tile-servers/using-openstreetmap-direct

│ ⛔ flutter_map
│ ⛔ flutter_map wants to help keep map data available for everyone.
│ ⛔ We use the public OpenStreetMap tile servers in our code examples & demo app,
│ ⛔ but they are NOT free to use by everyone.
│ ⛔ In an upcoming non-major release, requests to 'tile.openstreetmap.org' or
│ ⛔ 'tile.osm.org' will be blocked by default in release mode.
│ ⛔ Please review https://operations.osmfoundation.org/policies/tiles/ to see if
│ ⛔ your project is compliant with their Tile Usage Policy.
│ ⛔ For more information, see https://docs.fleaflet.dev/tile-servers/using-openstreetmap-direct.
│ ⛔ It describes in additional detail why we feel it is important to do this, how
│ ⛔ you can unblock the tile servers if your use-case is acceptable, the timeframes
│ ⛔ for this new policy, and how we're working to reduce requests without any extra
│ ⛔ work from you.