Dir auths should vote Sybil flag if they're zeroing out a relay's flags because Sybil

If the dir auths decide that you have too many relays on your IP address, they clear your history and don't vote any flags.

That can be pretty confusing for a relay operator, since there's no feedback loop ("why don't they like me") and also we haven't really documented our "no more than x relays per y kind of ip address" expectations.

This point came up most recently in #40243 (closed) where it turns out that we have new IPv6 Sybil restrictions.

We should brainstorm ways to help close the feedback loop.

One way would be for dir auths to vote the Sybil flag if that's the reason they're withholding flags. That way when the relay operator investigates, they at least have a chance to say "Sybil flag? What's that?"

(Note that in this version of the idea, I'm not proposing a new consensus method or really anything to do with the consensus -- first because that's extra complexity, but second because the relays would be omitted from the consensus anyway because they don't have enough Running votes.)