chore: rename task_config to benchmark_config (#1397)

# What does this PR do?

- This was missed from previous deprecation:
https://github.com/meta-llama/llama-stack/pull/1186
- Part of https://github.com/meta-llama/llama-stack/issues/1396

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
```
pytest -v -s --nbval-lax ./llama-stack/docs/notebooks/Llama_Stack_Benchmark_Evals.ipynb 
```

[//]: # (## Documentation)

docs/notebooks/Alpha_Llama_Stack_Post_Training.ipynb

@@ -3675,7 +3675,7 @@
         "    benchmark_id=\"llama3.2-3B-instruct:tax_eval\",\n",
         "    input_rows=eval_rows.rows,\n",
         "    scoring_functions=[\"braintrust::answer-similarity\"],\n",
-        "    task_config={\n",
+        "    benchmark_config={\n",
         "        \"type\": \"benchmark\",\n",
         "        \"eval_candidate\": {\n",
         "            \"type\": \"model\",\n",
@@ -6383,7 +6383,7 @@
         "    benchmark_id=\"Llama-3.2-3B-Instruct-sft-0:tax_eval\",\n",
         "    input_rows=eval_rows.rows,\n",
         "    scoring_functions=[\"braintrust::answer-similarity\"],\n",
-        "    task_config={\n",
+        "    benchmark_config={\n",
         "        \"type\": \"benchmark\",\n",
         "        \"eval_candidate\": {\n",
         "            \"type\": \"model\",\n",