MTV’s long-running reality competition program “The Challenge” will debut an “All Stars” season on ViacomCBS’ new streaming service Paramount Plus on April 1. The new nine-episode season will feature 22 returning “Challenge” competitors — yes, an entire season of true veterans — some of whom have just been seen on the most recent season, “The Challenge: Double Agents” on MTV, and some of whom are making their grand return after years away, all competing for a $500,000 grand prize.