Post-transplant cyclophosphamide, calcineurin inhibitor, and mycophenolate mofetil compared to anti-thymocyte globulin, calcineurin inhibitor, and methotrexate combinations as graft-versus-host disease prophylaxis post allogeneic stem cell transplantation from sibling and unrelated donors in patients with acute myeloid leukemia: a study on behalf of the Acute Leukemia Working Party of the European Society for Blood and Marrow Transplantation