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.
See https://github.com/libgit2/rugged/issues/909
rugged does not support setting custom extensions, and with
libgit 1.6, you need to set custom extensions to open a repository
with the partialclone extension.
