Since you are using the open driver, you have the last version available ( nvidia-open-driver-G06-signed-kmp-default 580.126.09 (580.126.18)) and no upgrade is going to happen after EOL, like you are not going to see further updates to the kernel.
You should still be able to upgrade to the 580.159.03 packages using the proprietary driver, just ensure that the following meta packages are installed in that version:
i+ | nvidia-driver-G06-kmp-meta | Meta package to select proprietary nvidia driver | package
i+ | nvidia-userspace-meta-G06 | Meta package to autoselect NVIDIA userspace packages | package
Anyway I would not use Leap 15.6 after EOL on a production system.