Related, downstream issue: streetcomplete/StreetComplete#2878

Use case: When displaying partially overlapping markers on a map, they are often hidden if their position is too close to each other. This often makes it impossible to click/read some markers, especially when the map is at max zoom already.

Proposed solution: Increase the maximum zoom level by 2-3 steps, e.g. to 22 or 23, so that applications can utilize higher zoom levels if they need to.