Apparently PS4 games are supported on PS5 as long as you use the DS4 controller, not the Dualsense, for which support is being investigated.
To set it up:
Plug your XIM into the PS5 and connect to DS4 controller. NOT the PS5 controller.
You might want to remap some of your keyboard and mouse controls: Here are mine:
WASD - movement
Mouse movement- aim with 'laser like' precision
Left mouse button - Insta kill enemy without warning
Right mouse button - zoom in to see instakill
Left Shift - T-bag DS4 user