Dispose of clients when disposing sharded client

src/Discord.Net.WebSocket/DiscordShardedClient.cs

@@ -382,6 +382,8 @@ namespace Discord.WebSocket
             {
                 if (disposing)
                 {
+                    foreach (var client in _shards)
+                        client?.Dispose();
                     _connectionGroupLock?.Dispose();
                 }