BnA doesn't support prevWinsMs

[fhoering]

Currently B&A only reads the prevWins field.

It seems like it didn’t move to the latest field prevWinMs that replaced prevWins in the spec: https://wicg.github.io/turtledove/#dom-biddingbrowsersignals

The Spec actually mentions a dedicated B&A section now: https://wicg.github.io/turtledove/#server-auction-browser-signals

The cleanest looks to align B&A browserSignals with the latest spec in order to have a consistent behavior.