Switch to fixed-dose ainuovirine, lamivudine, and tenofovir DF versus elvitegravir, cobicistat, emtricitabine, and tenofovir alafenamide in virologically suppressed people living with HIV-1: the 48-week results of the SPRINT trial, a multi-centre, randomised, double-blind, active-controlled, phase 3, non-inferiority trial