Here is the relevant devlog, looks like there are two settings. For the editor it’s “run/platforms/linuxbsd/prefer_wayland” in the editor settings. For the project it’s “display/display_server/driver.linux”.
If you are trying to run a built game with wayland then the build must have been exported with a beta version of Godot which seems unlikely, but you can use the command line option --display-driver wayland