Safety and Efficacy Outcomes in Pediatric Patients with Transfusion-Dependent β-Thalassemia (TDT) Receiving Betibeglogene Autotemcel (beti-cel; LentiGlobin for β-thalassemia) Gene Therapy in the Phase 3 Hgb-207 (Northstar-2) and Hgb-212 (Northstar-3) Studies