new syntax for RequiredValidUntil filter

[Cantor, Scott]

> I'd not argue with that, but (as always) I'd defer to Scott.  It would save a
> whole bunch of soul searching around the current mess.

I don't see the value of changing the filter name vs. changing a setting, either way it requires people to make a change and I think people understand "this setting is deprecated, convert to new setting" pretty easily. Either way it creates a fork in federation documentation, which I don't take lightly, but given no away around that fork, I think a simpler change is better.

I wouldn't be against auto-attaching the filter for the remote metadata source types in 4.0, I'm sure that would be a strengthening measure for many people. It's ugly code (I've screwed around with auto-attaching those in the parsers before) but it's our ugly code.

I guess one question: what are we gaining here? The issue isn't really that the current setting doesn't take durations, but that it's an outlier in taking a numeric value as seconds?

-- Scott