Update Together.ai default model to openai/gpt-oss-20b

The previous default (meta-llama/Meta-Llama-3.1-8B-Instruct-Turbo) is
no longer available as a serverless Together.ai model and now requires a
custom deployment. The new default is openai/gpt-oss-20b, one of
Together's recommended models for small & fast use-cases.

src/pipecat/services/together/llm.py

@@ -51,7 +51,7 @@ class TogetherLLMService(OpenAILLMService):
         Args:
             api_key: The API key for accessing Together.ai's API.
             base_url: The base URL for Together.ai API. Defaults to "https://api.together.xyz/v1".
-            model: The model identifier to use. Defaults to "meta-llama/Meta-Llama-3.1-8B-Instruct-Turbo".
+            model: The model identifier to use. Defaults to "openai/gpt-oss-20b".
 
                 .. deprecated:: 0.0.105
                     Use ``settings=TogetherLLMService.Settings(model=...)`` instead.
@@ -61,7 +61,7 @@ class TogetherLLMService(OpenAILLMService):
             **kwargs: Additional keyword arguments passed to OpenAILLMService.
         """
         # 1. Initialize default_settings with hardcoded defaults
-        default_settings = self.Settings(model="meta-llama/Meta-Llama-3.1-8B-Instruct-Turbo")
+        default_settings = self.Settings(model="openai/gpt-oss-20b")
 
         # 2. Apply direct init arg overrides (deprecated)
         if model is not None: