[data][train] Fix air_benchmark_xgboost_cpu_10

[raulchen]

Why are these changes needed?

air_benchmark_xgboost_cpu_10 is failing because actor pool autoscaling strategy is problematic and it makes data ingestion too slow. Changing to fixed sized actor pool to fix this issue.

The autoscaling issue will be fixed later, as it's not a commonly used feature. #41956

Related issue number

Closes #41859

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