bug#44808: Default to allowing password authentication on leaves users vulnerable

[raid5atemyhomework]

Hi guix users,

It strikes me that a better course of action here would be, rather than 
providing a warning that might not be noticed by the user, to remove the 
default and force people to explicitly put `password-authentication? #t` or 
`password-authentication? #f`.

That way if I have set up a headless server (possibly having a temporary 
keyboard/mouse/monitor during initial install, then forever logging in 
afterwards over intranet using my super secret password "raid5isnotagooddog"), 
with an existing `configuration.scm` that does not explicitly give the setting, 
I cannot accidentally lose access to my headless server by doing a random `guix 
pull && sudo guix system reconfigure configuration.scm` without noticing the 
warning.

Especially since there exists an `unattended-upgrades-service-type` which 
automates this `guix pull && sudo guix system reconfigure configuration.scm`, 
which makes changing this default ***VERY DANGEROUS*** in this use-case.  I'd 
rather I noticeably error out in this case.

Then later after a year give a "sane" default, after people who have depended 
on the existing `password-authentication? #t` have already  explicitly put the 
setting in their `configuration.scm`.

Thanks
raid5atemyhomework