A Phase 3, Open-label, Randomized Study to Compare the Efficacy and Safety of Luspatercept (ACE-536) vs Epoetin Alfa for the Treatment of Anemia Due to Revised International Prognostic Scoring System (IPSS-R) Very Low, Low, or Intermediate-Risk Myelodysplastic Syndrome (MDS) in Erythropoietin-stimulating Agent (ESA)-naive Participants who are Non-Transfusion Dependent (NTD)