The superuser is already used to check whether modules are installed
with demo data and create the notification.

It was not used for accessing the admin partner though, so in the case
where a user is logging in with the password "admin" and with no
access to `partner_admin` it's going to blow up the login process
despite no reason to.

Admittedly this is somewhat difficult to achieve in "standard" odoo,
it was only discovered because 2FA (odoo/odoo#33928) can reach this
point without having a "proper" session set up as that set up is
delayed until after the MFA step has succeeded while
`_admin_password_warn` is part of the redirection flow from the
initial login (user/password input).

closes odoo/odoo#53203

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>