handle weak-security repositories as unauthenticated

handle weak-security repositories as unauthenticated

APT can be forced to deal with repositories which have no security
features whatsoever, so just giving up on repositories which "just" fail
our current criteria of good security features is the wrong incentive.

Of course, repositories are better of fixing their setup to provide the
minimum of security features, but sometimes this isn't possible:
Historic repositories for example which do not change (anymore).

That also fixes problem with repositories which are marked as trusted,
but are providing only weak security features which would fail the
parsing of the Release file.

Closes: 827364