A 28-year-old woman with type 1 diabetes presents to the clinic for evaluation of skin lesions on her leg. They are not painful or itchy and have developed gradually over time. There is no history of previous trauma to the site either.On physical examination the skin changes have a central depression and raised irregular margin. The surrounding area is not red, tender, or indurated. They are shown in Figure below. Which of the following is the most likely diagnosis?