feat: Update changelog scraper to be able to open PRs to suggest adding releases from OCaml Platform GitHub projects automatically (#3217)

* update scraper to add releases
* allow marking changelog release as 'unstable'
* specify tags for release
* add ability to ignore releases
* fix
* remove 'description' on changelog release, add `github_release_tag` to disambiguate
* disambiguate by github release tag
* add some missing releases
* allow covering multiple github releases with one OCaml Changelog release announcement
* fix ability to ignore releases / scraper now doesn't re-scrape github tags of ignored releases

Author: sabine