Uprate SPM thresholds with a more realistic index

The Census Bureau defines SPM thresholds based on expenditures. This rises consistently faster than CPI, which is what we currently index to:

https://github.com/PolicyEngine/policyengine-us/blob/6525e72ffc89aca67e88ba6381dfa063be0123bc/policyengine_us/variables/household/income/spm_unit/spm_unit_spm_threshold.py#L10

Let's either identify a forecast of expenditures (ideally on the relevant categories: food, clothing, shelter, and utilities), or otherwise construct one based on CPI plus the typical delta.

Start with policyengine_us/variables/household/income/spm_unit/spm_unit_spm_threshold.py at the linked definition and review how the current CPI index is applied. Research expenditure forecasts for food, clothing, shelter, and utilities, or compare CPI with expenditure growth to define the alternative. Done means the SPM thresholds use a justified expenditure-based index or documented CPI adjustment.

Written by the indexing model from the issue text.