api: Mark User.isBillingAdmin as required, relying on server 5+, FL 73+

See "Feature level 73" from Zulip API changelog:

   https://zulip.com/api/changelog

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/api/model/model.dart

@@ -205,7 +205,7 @@ 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;
   final bool isBot;
   final int? botType; // TODO enum
   int? botOwnerId;

lib/api/model/model.g.dart

@@ -9,7 +9,7 @@ extension UserChecks on Subject<User> {
   Subject<String> get fullName => has((x) => x.fullName, 'fullName');
   Subject<String> get dateJoined => has((x) => x.dateJoined, 'dateJoined');
   Subject<bool> get isActive => has((x) => x.isActive, 'isActive');
-  Subject<bool?> get isBillingAdmin => has((x) => x.isBillingAdmin, 'isBillingAdmin');
+  Subject<bool> get isBillingAdmin => has((x) => x.isBillingAdmin, 'isBillingAdmin');
   Subject<bool> get isBot => has((x) => x.isBot, 'isBot');
   Subject<int?> get botType => has((x) => x.botType, 'botType');
   Subject<int?> get botOwnerId => has((x) => x.botOwnerId, 'botOwnerId');