When an invoice is paid, the button "Cancel" was not visible. For a better guidance, we now show that button.
Indeed, after having unreconcilied the invoice and the payment, the user should be able to cancel the invoice.
It also now allows to cancel wrongly encoded invoices with 0 as total (they were directly marked as paid, preventing
their cancellation)
Was PR #20470. Was task: https://www.odoo.com/web#id=34307&view_type=form&model=project.task&action=333&active_id=967&menu_id=4720