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/_static/llama-stack-spec.yaml

@@ -4357,13 +4357,13 @@ components:
           type: array
           items:
             type: string
-        task_config:
+        benchmark_config:
           $ref: '#/components/schemas/BenchmarkConfig'
       additionalProperties: false
       required:
         - input_rows
         - scoring_functions
-        - task_config
+        - benchmark_config
       title: EvaluateRowsRequest
     EvaluateResponse:
       type: object
@@ -6168,11 +6168,11 @@ components:
     RunEvalRequest:
       type: object
       properties:
-        task_config:
+        benchmark_config:
           $ref: '#/components/schemas/BenchmarkConfig'
       additionalProperties: false
       required:
-        - task_config
+        - benchmark_config
       title: RunEvalRequest
     Job:
       type: object