api [nfc]: Update TODO on User.isBillingAdmin

While is_billing_admin has become required since feature level 73, it then
got removed at feature level 363.

See "Feature level 73" and "Feature level 363":

   https://zulip.com/api/changelog

Author: PIG208

lib/api/model/model.dart

@@ -414,7 +414,7 @@ class User {
   // bool isOwner; // obsoleted by [role]; ignore
   // bool isAdmin; // obsoleted by [role]; ignore
   // bool isGuest; // obsoleted by [role]; ignore
-  bool? isBillingAdmin; // TODO(server-5)
+  bool? isBillingAdmin; // TODO(server-10)
   final bool isBot;
   final int? botType; // TODO enum
   int? botOwnerId;