compose: bump unbound version

docker-compose.yml

@@ -2,7 +2,7 @@ version: '2.1'
 services:
 
     unbound-mailcow:
-      image: mailcow/unbound:1.19.1
+      image: mailcow/unbound:1.20
       environment:
         - TZ=${TZ}
         - SKIP_UNBOUND_HEALTHCHECK=${SKIP_UNBOUND_HEALTHCHECK:-n}