Retire temporary `has_itin` aliases after `has_tin` migration

`policyengine-us` now has a clearer `has_tin` / `taxpayer_has_tin` naming layer, but it still keeps `has_itin` and `taxpayer_has_itin` as temporary compatibility aliases while downstream users migrate.

That is the right short-term tradeoff, but we should clean it up once the data side has a proper TIN imputation.

## Follow-up work

- Treat `has_tin` / `taxpayer_has_tin` as the canonical public variables.
- Migrate remaining call sites, tests, docs, and comments that still say "ITIN" when they really mean "TIN".
- Remove the temporary `has_itin` / `taxpayer_has_itin` aliases once downstream users and datasets have migrated.
- Recheck reform and state code that distinguishes SSN-only rules from broader TIN rules, so the final naming matches the legal concept.

## Blocker / dependency

This should happen after `policyengine-us-data` models TIN status directly rather than inferring it from `ssn_card_type != NONE`.

Related data issue: PolicyEngine/policyengine-us-data#712.