create-pri-oslist: fix release image URL

Apertis images are published as gzipped files with the extension .img.gz. However, URLs in the OS list lack the .gz suffix, leading to 404 errors when trying to flash release images (this doesn't affect daily images as the corresponding list is generated separately).

Once this is merged, the oslist files should be re-generated by running the following commands on the server:

$ create-rpi-oslist v2021.3
$ create-rpi-oslist v2023dev0.0