Lint sysusers file for fully locked system accounts

systemd v257 supports and recommends fully locked system accounts[1][2][3]:

systemd-sysusers is now able to create fully locked user accounts. For compatibility it so far created accounts with a locked (i.e. invalid) password, but not marked locked as a whole. With the new "!" modifier for "u" lines, it is now possible to create fully locked accounts. The distinction between accounts with a locked password and fully locked accounts is relevant when considering non-password forms of authentication, i.e. SSH and such. It is strongly recommended to make use of this new feature for almost all system accounts, since they usually do not require (and should not permit) interactive logins. All of systemd's own system users have been changed to be marked as fully locked.

1. https://github.com/systemd/systemd/blob/25a306f6c4d9b8596e25d7771802f005ebae91ba/NEWS#L822-L832
2. https://mastodon.social/@pid_eins/113395418561365864
3. https://www.freedesktop.org/software/systemd/man/latest/sysusers.d.html#u

I think it would be good to link for this for future packages aswell