Apparently Rockchip doesn’t keep their GitHub updated anymore but you can find the official 6.1 branch over on Gitlab: Files · develop-6.1 · RK3588_Linux / rk / kernel · GitLab
Personally I think there’s two ways to go either fully mainline (but there’s issues with complete support for HW accelerations)
Or with the vendor kernel which seems to have active (enough) development and full HW accelerations. I was also able to compile it on modern Ubuntu Jammy LTS but I do need to test it a bit more