Automate adding the contributors of a release to its release note

Since RN-70, the contributors of a release are stored in a dedicated "Contributors" entry of its release note on xwiki.org, and rendered by the releasenotecontributors macro. The release manager still has to run list_contributors.sh, clean the result (bots such as Renovate, Copilot or Claude co-authors appear in it) and paste it through the "Add contributors" button, by hand, for every release.

Add a script to the release scripts that:

• computes the contributors between two versions with list_contributors.sh and drops the bots
• checks that the release note of the end version exists and is for that version
• creates the "Contributors" entry of that release note, or refuses to replace an existing one (showing the differences) unless asked to
• supports a dry run, which needs no credentials

See also https://forum.xwiki.org/t/automate-the-contributors-list-during-the-release/18663