public Set<Integer> unackedVoters() {
        return unackedVoters;
    }
Get the set of voters which have yet to acknowledge the resignation.
This node will send `EndQuorumEpoch` requests to this set until these
voters acknowledge the request or we transition to another state.

@return the set of unacknowledged voters
raft/src/main/java/org/apache/kafka/raft/ResignedState.java