Support Rails 8: build_batch_orders now takes a cursor argument

[mykhi]

Rails 8 changed the private ActiveRecord::Batches#build_batch_orders signature from (order) to (cursor, order) to support composite primary keys:

# Rails 8.0
def build_batch_orders(cursor, order)
  cursor.zip(Array(order)).map do |column, order_|
    [column, order_ || DEFAULT_ORDER]
  end
end

The IS_RAILS_7_1_PLUS branch still called the one-arg form, so every pluck_in_batches / pluck_each call on an ActiveRecord relation raised:

ArgumentError: wrong number of arguments (given 1, expected 2)

This adds an IS_RAILS_8_PLUS branch that passes Array(primary_key) as the cursor. Behaviour is unchanged — both forms produce { primary_key => :asc }. Rails < 8 paths are untouched.

Verified against maxio-com/chargify on the Rails 8 upgrade branch (fnd-840-rails-8-upgrade, PR #30000), which was failing 22 examples on this error:

• spec/jobs/batch/backfill_customer_maxioid_spec.rb — 25 examples, 0 failures
• spec/lib/jobs/batch/proforma_invoice_pdf_zip_export_spec.rb — 17 examples, 0 failures

Also exercised the call shapes used across that codebase (single primary key, multi-column, non-primary-key column, string column names, pluck_each, and a relation with a pre-existing order) — results and ascending ordering match Rails 7.2.