Update foundational examples to use "user" role

Use system_instruction on LLM service constructors instead of adding
system messages to LLMContext. Messages added to context now use
"user" role.

examples/foundational/24-user-mute-strategy.py

@@ -143,7 +143,7 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
         # Kick off the conversation with a weather-related prompt
         context.add_message(
             {
-                "role": "system",
+                "role": "user",
                 "content": "Ask the user what city they'd like to know the weather for.",
             }
         )