Create proper GitHub releases with the xwiki release script

GitHub makes a distinction between Git tag and releases. Both are displayed in the same UI, however a Github releases can be published manually (see: https://help.github.com/en/articles/creating-releases) or automatically (see: https://help.github.com/en/articles/automation-for-release-forms-with-query-parameters).
Right now the only way in Github to get a "release notification" (see: https://help.github.com/en/articles/watching-and-unwatching-releases-for-a-repository) is by creating Github releases, pushing Git tags is not enough.
So it might be interesting that our release script creates automatically a Github release whenever we release a new version, at least to push the right notifications on Github for people who follows our repo.