followup questions … after upgrading everything I did:
$ sudo systemctl disable mattermost.service
Removed /etc/systemd/system/multi-user.target.wants/mattermost.service.
Removed /etc/systemd/system/mysql.service.wants/mattermost.service.
follower by sudo do-release-upgrade (from Ubuntu 20.04 to 22.04).
Which was successfully finished.
$ sudo systemctl enable mattermost
Created symlink /etc/systemd/system/multi-user.target.wants/mattermost.service → /lib/systemd/system/mattermost.service.
and noticed, that … mysql isn’t anymore wanted or required for mattermost service?