A 58-year-old man presents with a 2-year history of bilateral lower limb claudication that worsens with standing and walking, but improves with sitting and forward bending. MRI of the lumbar spine shows degenerative changes with central canal stenosis at L4–L5 and L5–S1 levels. He has normal ankle reflexes and no focal motor deficit. Conservative management with NSAIDs and physical therapy has failed. Which of the following is the most appropriate surgical approach for this patient?