@Override
    public void validate(ChangeContext context) {
        for (Map.Entry<String, ContentCluster> currentEntry : context.previousModel().getContentClusters().entrySet()) {
            ContentCluster nextCluster = context.model().getContentClusters().get(currentEntry.getKey());
            if (nextCluster == null) continue;
            validateContentCluster(currentEntry.getValue(), nextCluster).forEach(context::require);
        }
    }
Returns any change to the indexing mode of a cluster.

@author hmusum
@author bjorncs
config-model/src/main/java/com/yahoo/vespa/model/application/validation/change/IndexingModeChangeValidator.java