Due to elastic#41185 datafeeds created prior to 7.0 and not updated since
then have unparseable authorization headers in 8.x. In 8.0-8.3 this
could easily be a non-issue, as such datafeeds were likely forgotten
leftovers and never run. Even if it was a problem, only the datafeed
of interest would need updating with any urgency.

Due to elastic#87884 datafeeds with authorization headers older than 7.0
prevent _all_ datafeeds being listed in 8.4 and 8.5. This in turn
breaks the anomaly detection job management section of the ML UI.

The problem is alleviated by elastic#92168 and fixed by elastic#92221, but we
should warn users that the problem exists in 8.4.0-8.5.3 inclusive.

Co-authored-by: István Zoltán Szabó <istvan.szabo@elastic.co>

…into ml_dfeed_auth_known_issue