Meanwhile, thanks to another Audi E-tron GT user, I received a solution:
1) Reset the Car's MMI. (hold down the MMI button on the center console until restart)
2) Select CarPlay connection on the Audi MMI. Reconnect iPhone with the Car's WiFi, which Apple with the update to iOS 17.2.1 disconnected.
3) Set WiFi Preferences on iPhone as before. Unfortunately, it seems that Apple with the Update changes the settings.
4) This is not the first time that after an Apple Update the presets are completely wrong.
5) I hope my Audi keeps the iPhone automatically connected to Carplay when starting the car.
6) The Audi eTron GT is simply awesome on the road and full-electric!