download: Standardize filenames

Current filename examples (only looking at files linked from the site):

• freedombox-bullseye-free_all-arm64.img.xz
• freedombox-testing_latest_all-arm64.img.xz
• freedombox-bullseye-free_all-amd64.img.xz
• freedombox-testing_latest_all-amd64.img.xz
• freedombox-bullseye-free_a20-olinuxino-lime2-armhf.img.xz
• freedombox-testing_latest_a20-olinuxino-lime2-armhf.img.xz
• freedombox-stable-free_pioneer_a20-olinuxino-lime2-armhf.img.xz
• freedombox-bullseye-nonfree_raspberry64-arm64.img.xz
• freedombox-testing_latest_raspberry64-arm64.img.xz

Proposals:

• Include the version of FreedomBox that is included in the image.
• Drop the "latest" tag for testing images.
• Drop the architecture tag if there is no ambiguity.
• Drop the free/nonfree tag (this was done for testing already)
• For generic images, drop "all" and just have the architecture name. If needed, we could distinguish UEFI and legacy boot images (freedom-maker#211).