refactor: update import statement for ServerBuilder to Builder

Author: CodeWithKyrian

src/Server.php

@@ -12,7 +12,7 @@
 namespace Mcp;
 
 use Mcp\Server\Handler\JsonRpcHandler;
-use Mcp\Server\ServerBuilder;
+use Mcp\Server\Builder;
 use Mcp\Server\Transport\TransportInterface;
 use Psr\Log\LoggerInterface;
 use Psr\Log\NullLogger;