It may not be the best place for that (gha.py would have been better), but it's the shorter / faster way to do it for now.
Moreover it now uses logging for writing the group. The logger format has been updated for this to work. This was done to fix issues on GitHub Action logs, where groups were declared after the logs.
- Move frontmatter-related operation from Product to ProductFrontmatter. This makes more senses, as we are manipulating different files / kind of data.
- Use Product directly to load old versions.
