Brian Steinberg Senior TV EditorBehind the scenes, the TV news biz is reeling as top talent exit rarefied posts for new ventures, worn out after an intense year covering the Trump administration and the rise of the coronavirus.Four of TV’s top news programs — ABC’s “Good Morning America,” “CBS Evening News,” “NBC Nightly News” and the fourth hour of NBC’s “Today” morning franchise — are on the hunt for executive producers.