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.html

@@ -6355,7 +6355,7 @@
                             "type": "string"
                         }
                     },
-                    "task_config": {
+                    "benchmark_config": {
                         "$ref": "#/components/schemas/BenchmarkConfig"
                     }
                 },
@@ -6363,7 +6363,7 @@
                 "required": [
                     "input_rows",
                     "scoring_functions",
-                    "task_config"
+                    "benchmark_config"
                 ],
                 "title": "EvaluateRowsRequest"
             },
@@ -9248,13 +9248,13 @@
             "RunEvalRequest": {
                 "type": "object",
                 "properties": {
-                    "task_config": {
+                    "benchmark_config": {
                         "$ref": "#/components/schemas/BenchmarkConfig"
                     }
                 },
                 "additionalProperties": false,
                 "required": [
-                    "task_config"
+                    "benchmark_config"
                 ],
                 "title": "RunEvalRequest"
             },