Boeing Co.’s disastrous begin to 2024 is spilling over to airways and their passengers as manufacturing delays exacerbate a scarcity of single-aisle jets.United Airways Holdings Inc., Southwest Airways Co. and Ryanair Holdings Plc are among the many firms scrambling to answer decreased deliveries from Boeing because the planemaker focuses on fixing high quality lapses uncovered by the Jan. 5 accident on an Alaska Airways flight.