docs(java-quickstart): update tool registration method in java client quickstart (modelcontextprotocol#465)

Update client.mdx to reflect breaking change replacing the  `.defaultTools()` by `.defaultToolCallbacks()` for registering MCP tools.

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

Author: tzolov

docs/quickstart/client.mdx

@@ -919,17 +919,22 @@ The chatbot is implemented using Spring AI's ChatClient with MCP tool integratio
 ```java
 var chatClient = chatClientBuilder
     .defaultSystem("You are useful assistant, expert in AI and Java.")
-    .defaultTools((Object[]) mcpToolAdapter.toolCallbacks())
+    .defaultToolCallbacks((Object[]) mcpToolAdapter.toolCallbacks())
     .defaultAdvisors(new MessageChatMemoryAdvisor(new InMemoryChatMemory()))
     .build();
 ```
 
+<Warning>
+Breaking change: From SpringAI 1.0.0-M8 onwards, use `.defaultToolCallbacks(...)` instead of `.defaultTool(...)` to register MCP tools.
+</Warning>
+
 Key features:
 - Uses Claude AI model for natural language understanding
 - Integrates Brave Search through MCP for real-time web search capabilities
 - Maintains conversation memory using InMemoryChatMemory
 - Runs as an interactive command-line application
 
+
 ### Build and run
 
 ```bash