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/source/building_applications/evals.md

@@ -51,7 +51,7 @@ response = client.eval.evaluate_rows(
     benchmark_id="meta-reference::mmmu",
     input_rows=eval_rows,
     scoring_functions=["basic::regex_parser_multiple_choice_answer"],
-    task_config={
+    benchmark_config={
         "type": "benchmark",
         "eval_candidate": {
             "type": "model",
@@ -109,7 +109,7 @@ response = client.eval.evaluate_rows(
     benchmark_id="meta-reference::simpleqa",
     input_rows=eval_rows.rows,
     scoring_functions=["llm-as-judge::405b-simpleqa"],
-    task_config={
+    benchmark_config={
         "type": "benchmark",
         "eval_candidate": {
             "type": "model",
@@ -158,7 +158,7 @@ response = client.eval.evaluate_rows(
     benchmark_id="meta-reference::simpleqa",
     input_rows=eval_rows.rows,
     scoring_functions=["llm-as-judge::405b-simpleqa"],
-    task_config={
+    benchmark_config={
         "type": "benchmark",
         "eval_candidate": {
             "type": "agent",

docs/source/building_applications/evaluation.md

@@ -19,7 +19,7 @@ response = client.benchmarks.register(
 # Run evaluation
 job = client.eval.run_eval(
     benchmark_id="my_eval",
-    task_config={
+    benchmark_config={
         "type": "app",
         "eval_candidate": {"type": "agent", "config": agent_config},
     },

docs/source/references/evals_reference/index.md

@@ -87,7 +87,7 @@ response = client.eval.evaluate_rows(
     benchmark_id="meta-reference::mmmu",
     input_rows=eval_rows,
     scoring_functions=["basic::regex_parser_multiple_choice_answer"],
-    task_config={
+    benchmark_config={
         "type": "benchmark",
         "eval_candidate": {
             "type": "model",
@@ -145,7 +145,7 @@ response = client.eval.evaluate_rows(
     benchmark_id="meta-reference::simpleqa",
     input_rows=eval_rows.rows,
     scoring_functions=["llm-as-judge::405b-simpleqa"],
-    task_config={
+    benchmark_config={
         "type": "benchmark",
         "eval_candidate": {
             "type": "model",
@@ -195,7 +195,7 @@ response = client.eval.evaluate_rows(
     benchmark_id="meta-reference::simpleqa",
     input_rows=eval_rows.rows,
     scoring_functions=["llm-as-judge::405b-simpleqa"],
-    task_config={
+    benchmark_config={
         "type": "benchmark",
         "eval_candidate": {
             "type": "agent",