This endpoint allows bulk cancellation of invoices. Each invoice is processed independently,
enabling partial success scenarios where some invoices may be cancelled while others fail validation.

Conditions:

• Only invoices with status 'Approved' and payment status 'Unpaid' can be cancelled.

Effects of Cancellation:

• Invoice status is updated to 'Cancelled'.
• All related Invoice Line Items are Cancelled.
• Billing Header Rollup - Total Invoice Amount and Total Pending Amount are updated.