fix: deny all subscribes to '#' topics in the default acl.conf

apps/emqx_authz/etc/acl.conf

@@ -26,3 +26,7 @@
 {allow, {username, "^dashboard?"}, subscribe, ["$SYS/#"]}.
 
 {allow, {ipaddr, "127.0.0.1"}, all, ["$SYS/#", "#"]}.
+
+{deny, all, subscribe, ["$SYS/#", {eq, "#"}]}.
+
+{allow, all}.