Make regional admin borders solid instead of dashed

Due to the coastline paradox, dashed lines for borders don't work
well on low-zoom: strongly curved borders appear solid instead of
dashed. Also, solid dashes generally are too busy for the crowded
z5-7.

This PR makes regional boundaries (admin_level=4) solid instead of
dashed up to z9.

To prevent the borders from getting too strong, they are also made
thinner.

admin.mss

@@ -65,28 +65,28 @@ overlapping borders correctly.
       line-color: @admin-boundaries;
       line-join: bevel;
       line-width: 0.4;
-      line-dasharray: 4,3;
       line-clip: false;
     }
     [zoom >= 5] {
-      background/line-width: 0.6;
-      line-width: 0.6;
+      background/line-width: 0.5;
+      line-width: 0.5;
     }
     [zoom >= 6] {
-      background/line-width: 0.8;
-      line-width: 0.8;
+      background/line-width: 0.6;
+      line-width: 0.6;
     }
     [zoom >= 7] {
-      background/line-width: 1;
-      line-width: 1;
+      background/line-width: 0.7;
+      line-width: 0.7;
     }
     [zoom >= 9] {
-      background/line-width: 1.8;
-      line-width: 1.8;
+      background/line-width: 1;
+      line-width: 1;
     }
     [zoom >= 10] {
       background/line-width: 2.5;
       line-width: 2.5;
+      line-dasharray: 4,3;
     }
     [zoom >= 12] {
       background/line-width: 3;