static RetryRule onServerErrorStatus(Backoff backoff) {
        return builder().onServerErrorStatus().thenBackoff(backoff);
    }
Returns a newly created {@link RetryRule} that will retry with the specified {@link Backoff} if the
class of the response status is {@link HttpStatusClass#SERVER_ERROR}.
core/src/main/java/com/linecorp/armeria/client/retry/RetryRule.java