api: notification: make the 'mail-to-root' target visible to any user

api: notification: make the 'mail-to-root' target visible to any user

Since the target does not require Mapping.Use, it should also be
visible and testable by all users.

Short explanation why the 'mail-to-root' is exempt from priv checks:

To ensure backwards compatibility, the 'mail-to-root' target does not
require the `Mapping.Use` privs. This is needed due to the fact that
this target is used as a fallback in case no other target is
configured for an event. For instance, the /node/<name>/apt/update API
call only requires Sys.Modify for the node, but it can also send a
notification. If we were to require Mapping.Use, we could break the
apt/update API compat in the case that a notification shall be sent,
but without any configured notification target (which will then
default to 'mail-to-root').

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>