Use the channel with Welcome new user message as birthday message channel

DeukBot4/Database/BirthdayHandler.cs

@@ -139,7 +139,7 @@ namespace DeukBot4.Database
                     eb.Description  = $"It's <@!{guildUser.Id}>'s {age}th birthday!! Have a good one!";
                     eb.ThumbnailUrl = guildUser.GetAvatarUrl();
 
-                    guild.DefaultChannel.SendMessageAsync("", embed: eb.Build());
+                    guild.SystemChannel.SendMessageAsync("", embed: eb.Build());
                 }
             }
         }