Update Gradium endpoints to region-neutral URLs

Drop the EU-region default from the STT/TTS WebSocket URLs in favor of
the generic api.gradium.ai endpoint, and remove the explicit overrides
from the examples so they pick up the new defaults.

examples/transcription/transcription-gradium.py

@@ -51,7 +51,6 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
 
     stt = GradiumSTTService(
         api_key=os.environ["GRADIUM_API_KEY"],
-        api_endpoint_base_url="wss://us.api.gradium.ai/api/speech/asr",
         settings=GradiumSTTService.Settings(
             language=Language.EN,
             delay_in_frames=8,