Display SecTXT unknown_field as informational

[bwbroersma]

Currently internet.nl doesn't uses the ._notifications from SecTXT. So custom / wrongly spelled fields are only spotted if they are required.

I think all _notifications should be ℹ️ informational.

[{'code': 'unknown_field', 'message': 'security.txt contains an unknown field. Field "new-header" is either a custom field which may not be widely supported, or there is a typo in a standardised field name.', 'line': 3}]

unknown_field currently is the only code that is possible in _notifications (see code).