fix: missing minZoom on towns

On towns with a population, it would fall back to the default places zoom of 7. This sets a mininum of 8, similar to other place types.

Also sets fallback population of cities to 50k instead of 5k

ref protomaps#507

Author: jgillich

tiles/src/main/java/com/protomaps/basemap/layers/Places.java

@@ -90,7 +90,11 @@ public Places(CountryCoder countryCoder) {
     rule(
       with("place", "city"),
       without("population"),
-      use("population", 5000),
+      use("population", 50000),
+      use("minZoom", 8)
+    ),
+    rule(
+      with("place", "town"),
       use("minZoom", 8)
     ),
     rule(