v8.12.1

Fixes

Document required fields on several API resource responses

Several resource response schemas were missing required field lists,
even though the fields are always present in the actual response.
Added accurate required arrays (and fixed a few hardcoded docs
examples that predated them) for: payment, bank_details_lookup,
billing_request_template, currency_exchange_rate, mandate_import,
mandate_import_entry, negative_balance_limit, payer_authorisation,
payout, payout_item, refund, scenario_simulator,
scheme_identifier, tax_rate, verification_detail and logo.

This is a documentation-only change; API behaviour is unchanged.