After upgrade from 15.3 (in 2 steps) now on 17.2, but getting vmlinuz 5.15... in apt updates rather than vmlinuz 6.8

Interesting. Well, you can set it to

sudo nano /etc/default/grub

GRUB_DEFAULT="Zorin, with Linux 6.8.0-52-generic"
Tap ctl+o to overwrite, then enter to save current configuration, then ctl+x to exit the editor.
Now, run

sudo update-grub

Reboot and test if it boots to 6.8

Alternatively

You can use this simple technique to set it to the Later kernel, by following the same steps, but doing so booted into the 6.8 kernel: