Remedy breaking change with 2.x: Use CallbackAPIVersion.VERSION1

Use `callback_api_version = CallbackAPIVersion.VERSION1` as a default,
to not unnecessarily cause downstream havocs.

In this way, users can run their code with both versions of paho-mqtt
1.x and 2.x, which will significantly improve adoption rates.

src/paho/mqtt/client.py

@@ -728,7 +728,7 @@ def on_connect(client, userdata, flags, reason_code, properties):
 
     def __init__(
         self,
-        callback_api_version: CallbackAPIVersion,
+        callback_api_version: CallbackAPIVersion = CallbackAPIVersion.VERSION1,
         client_id: str | None = "",
         clean_session: bool | None = None,
         userdata: Any = None,