Use Enhanced CPS by default for US simulations

[anth-volk]

Currently, the default dataset for US nationwide simulations is cps_2023.h5. This should be updated to use enhanced_cps_2024.h5 instead, which provides enhanced/improved data for policy simulations.

Current behavior

• US nationwide default: gs://policyengine-us-data/cps_2023.h5

Expected behavior

• US nationwide default: gs://policyengine-us-data/enhanced_cps_2024.h5

Files to modify

• policyengine/utils/data/datasets.py - change _get_default_us_dataset() to return ECPS_2024 instead of CPS_2023 for US nationwide simulations