chore(deps): update policyengine-uk to 2.88.20

[policyengine]

Summary

Update PolicyEngine UK from 2.88.14 to 2.88.20 in the simulation API runtime.

What changed (2.88.14 -> 2.88.20)

Changed

• Remove redundant uprating metadata and class-level aggregation metadata from variables that already define their computation explicitly.
• Make the CPS expanded Marriage Allowance reform use the explicit would_claim_marriage_allowance input instead of formula-time randomness.

Fixed

• • Include council_tax_benefit in household_benefits, gov_spending, hbai_household_net_income, and pre_budget_change_household_benefits. Previously CTR was absent from these aggregates, so abolishing council tax via gov.contrib.abolish_council_tax refunded the gross billed amount to households (and removed gross revenue from the government balance) rather than the net out-of-pocket amount, overstating household savings by about £4 billion in aggregate.

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