[IMP] stock, purchase_stock: remove security lead time from picking deadline

Remove the Purchase Security Lead Time from the computing of the
purchase order's picking deadline to better reflect that the shipment is
actually meant to arrive earlier than date + security lead time.

Also makes sure that receipts and subsequent pickings (multi-step
reception) are planned the earliest possible, not taking the purchase
security lead time into account.

Task-2656397

Part-of: odoo/odoo#78199