api: Mark realmWebPublicAccessEnabled as required, relying on server 5+, FL 116+

See "Feature level 116" from Zulip API changelog:

   https://zulip.com/api/changelog

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

Author: PIG208

lib/api/route/realm.dart

@@ -48,7 +48,7 @@ class GetServerSettingsResult {
   final String realmName;
   final String realmIcon;
   final String realmDescription;
-  final bool? realmWebPublicAccessEnabled; // TODO(server-5)
+  final bool realmWebPublicAccessEnabled;
 
   GetServerSettingsResult({
     required this.authenticationMethods,