[Fix] Entitlements starts_at is nullable (#3034)

* i wonder if it was just as simple as that

* nope

* nope x2

* nope x3

src/Discord.Net.WebSocket/Entities/AppSubscriptions/SocketEntitlement.cs

@@ -68,9 +68,7 @@ public class SocketEntitlement : SocketEntity<ulong>, IEntitlement
         ApplicationId = model.ApplicationId;
         Type = model.Type;
         IsConsumed = model.IsConsumed.GetValueOrDefault(false);
-        StartsAt = model.StartsAt.IsSpecified
-            ? model.StartsAt.Value
-            : null;
+        StartsAt = model.StartsAt;
         EndsAt = model.EndsAt.IsSpecified
             ? model.EndsAt.Value
             : null;