feat: add young_player_system feature flag to disable Young player system (#2445)

Adds an opt-in 'youngPlayerSystem.enabled' server setting (default true) that, when set to false, disables the Young player system server-wide:

- Account.Young and PlayerMobile.Young getters short-circuit to false.

- New characters no longer receive Young status or a NewPlayerTicket.

- All downstream Young checks (notoriety beneficial-action restriction, CheckYoungProtection, stealing penalties, YoungDeathTeleport, death-item movement, poison immunity, '(Young)' name suffix, CanLogout, renounce-young keyword/gump, BaseCreature.OnDeath fame penalty, OnLogin time-remaining message) become inert.

Setters are intentionally left untouched so serialized account/player flags round-trip cleanly when the setting is later re-enabled.

Projects/UOContent/Accounting/Account.cs

@@ -10,6 +10,7 @@ using Server.Misc;
 using Server.Mobiles;
 using Server.Multis;
 using Server.Network;
+using Server.Systems.FeatureFlags;
 
 namespace Server.Accounting;
 
@@ -250,7 +251,7 @@ public partial class Account : IAccount, IComparable<Account>
     /// </summary>
     public bool Young
     {
-        get => !GetFlag(1);
+        get => ContentFeatureFlags.YoungPlayerSystem && !GetFlag(1);
         set
         {
             SetFlag(1, !value);