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