Add helper to publish phosh-next

Triggered by https://gitlab.gnome.org/World/Phosh/phoc/-/merge_requests/301#note_1305461

Likely a bit too phosh specific. Would also benefit from being done in python so we can query gitlab-ci to automatically drop merged MRs (which is currently manual), we could then also automatically fetch the list of MRs by using a tag in gitlab rather than having it in the local config and have the MRs title in d/changelog.