chore: Google AI - suggest users to switch to Google GenAI (#2106)

* chore: Google AI - suggest users to switch to Google GenAI

* docs reference

Author: anakin87

integrations/google_ai/src/haystack_integrations/components/generators/google_ai/chat/gemini.py

@@ -174,6 +174,11 @@ def __init__(
         :param streaming_callback: A callback function that is called when a new token is received from the stream.
             The callback function accepts StreamingChunk as an argument.
         """
+        soft_deprecation_msg = (
+            "This component uses a deprecated SDK. We recommend using the GoogleGenAIChatGenerator instead. "
+            "Documentation is available at https://docs.haystack.deepset.ai/docs/googlegenaichatgenerator."
+        )
+        logger.warning(soft_deprecation_msg)
 
         genai.configure(api_key=api_key.resolve_value())
         _check_duplicate_tool_names(tools)