Improve generation of GitHub release text

[yrodiere]

See https://github.com/hibernate/hibernate-release-scripts/blob/main/github-release.sh
See https://hibernate.atlassian.net/browse/HHH-19347

Right now, the text of GitHub releases is either a very simple placeholder, or a manually written file.

It's missing in particular:

1. A way to inject variables (e.g. the "series" / "version family") so that we don't have to update links manually in that file.
2. A way to generate a list of issues, like we usually do for Hibernate Reactive, e.g. https://github.com/hibernate/hibernate-reactive/releases/tag/3.0.0 . An alternative could be to link to the website, but it currently has lists of issues for each version only for projects using Jira, not for those using GitHub issues.

cc @sebersole @DavideD