generate-changes: get index information from releases
Instead of maintaining a manual list of indexes that needs to be kept up-to-date, use the information from alpinelinux.org/releases.json to determine what indexes to check.
This file is updated after every release, so we do not need to remember to update the list of indexes for the mirror-status to show.
What still can be improved is that the information in releases.json is also used to determine what arches and branches to show.
Fixes #2 (closed)