Update Cerebras from Llama 3.1 to 3.3 (#645)

# What does this PR do?

Cerebras is rolling out support for llama 3.3 70b and deprecating llama
3.1 70b. This PR updates the documentation, config, and internal mapping
to reflect this change.

cc: @ashwinb @raghotham

llama_stack/providers/remote/inference/cerebras/cerebras.py

@@ -41,8 +41,8 @@ model_aliases = [
         CoreModelId.llama3_1_8b_instruct.value,
     ),
     build_model_alias(
-        "llama3.1-70b",
-        CoreModelId.llama3_1_70b_instruct.value,
+        "llama-3.3-70b",
+        CoreModelId.llama3_3_70b_instruct.value,
     ),
 ]