A Phase 1/2, Randomized, Controlled, Observer-blind, Multicentre Study to Evaluate the Safety, Reactogenicity, and Immunogenicity of GSK Biologicals' Investigational Respiratory Syncytial Virus (RSV)/Human Metapneumovirus (hMPV) Combination and Investigational hMPV Vaccines When Administered Intramuscularly According to a Single Dose Schedule in Younger Adults ≥18 to ≤49 Years and Older Adults Aged ≥60 to ≤80 Years | Arctuva