Rename capital threshold labels to capital limits

[MaxGhenis]

Summary

• rename the DWP 16,000 GBP cutoff parameter paths from capital_limit to limit
• rename the tariff-income start parameter paths from lower_threshold to threshold
• update Universal Credit, Housing Benefit, and Income Support call sites to use the new shorter names

Testing

• uvx ruff check policyengine_uk/variables/gov/dwp/universal_credit/is_uc_eligible.py policyengine_uk/variables/gov/dwp/universal_credit/income/uc_tariff_income.py policyengine_uk/variables/gov/dwp/income_support_eligible.py policyengine_uk/variables/gov/dwp/income_support_tariff_income.py policyengine_uk/variables/gov/dwp/housing_benefit/housing_benefit_eligible.py policyengine_uk/variables/gov/dwp/housing_benefit/applicable_income/housing_benefit_tariff_income.py
• uv run policyengine-core test policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/is_UC_eligible.yaml policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/income/uc_unearned_income.yaml policyengine_uk/tests/policy/baseline/finance/benefit/family/income_support/income_support_capital.yaml policyengine_uk/tests/policy/baseline/finance/benefit/family/housing_benefit/housing_benefit_capital.yaml -c policyengine_uk