private static Result checkLegacyZeroBucketsStoredOnContentNode(long lastBuckets) {
        if (lastBuckets != 0) {
            return disallow("The storage node manages %d buckets".formatted(lastBuckets));
        }
        return allow();
    }
Checks if a node can be upgraded.

@author Haakon Dybdahl
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/NodeStateChangeChecker.java