Cleanup unused code in update.rb: all methods are now handled by custom scripts. This should fix errors encountered in update.yml workflow (such as https://github.com/endoflife-date/release-data/actions/runs/7279373761) since the dependency to psych has been removed.
This also reformat and update the update.yml workflow so that two workflows targeting the same branch cannot run concurrently. This change has been done because in such cases the last one always fail (changes cannot be commited because the previous workflow already update the branch).
Fixes#260.
The main reason for doing this is to have some common code between scripts, so that it is easier to change the JSON schema globally and normalize a few things (such as release order).
The Ruby code was kept as is so we can quickly roll back if necessary.
