Should be under Display → Lock screen → Double-line clock, but well, it isn’t.
Seems like Google added that option in Android 12 after the general release, but only for Pixels, and from what I can tell it’s supported on other devices starting with Android 13. I believe I’ve had this option on 12 on the FP4 as well, so there might still be hope that it’s going to get backported before Android 13 ships 
Oh and welcome to the community BTW 