Prebid 10: phase out bidder request.{}.userID?

[patmmccann]

Type of issue

I propose adding a deprecation warning in the prebid 9 release notes and in the reviewer guidelines indicating the intent to retire the bidderRequest.{}.userId object in prebid 10. eids seems to always be preferred as these objects become more complex and we've duplicated much of the weight of the object between the two.