public void acknowledgeResignation(int voterId) {
        if (!voters.contains(voterId)) {
            throw new IllegalArgumentException("Attempt to acknowledge delivery of `EndQuorumEpoch` " +
                "by a non-voter " + voterId);
        }
        unackedVoters.remove(voterId);
    }
Invoked after receiving a successful `EndQuorumEpoch` response. This
is in order to prevent unnecessary retries.

@param voterId the ID of the voter that send the successful response
raft/src/main/java/org/apache/kafka/raft/ResignedState.java