Update cartesia.py to use the new model pointer sonic instead of sonic-english

We are now using `sonic` as a pointer to the latest stable release (https://docs.cartesia.ai/build-with-sonic/models#continuous-updates). sonic-english will forever point to `sonic-2024-10-19`, which is already out of date.

src/pipecat/services/cartesia.py

@@ -88,7 +88,7 @@ class CartesiaTTSService(WordTTSService, WebsocketService):
         voice_id: str,
         cartesia_version: str = "2024-06-10",
         url: str = "wss://api.cartesia.ai/tts/websocket",
-        model: str = "sonic-english",
+        model: str = "sonic",
         sample_rate: int = 24000,
         encoding: str = "pcm_s16le",
         container: str = "raw",
@@ -329,7 +329,7 @@ class CartesiaHttpTTSService(TTSService):
         *,
         api_key: str,
         voice_id: str,
-        model: str = "sonic-english",
+        model: str = "sonic",
         base_url: str = "https://api.cartesia.ai",
         sample_rate: int = 24000,
         encoding: str = "pcm_s16le",