BUGFIX: Ignore lastModification in favor of creation and publicationDates ZombieDetector.php

[mficzel]

The last lastModification field is updated implicitly when any of the following fields changes.

aus NodeData.php

    /**
     * @var \DateTime
     * @Gedmo\Timestampable(on="update", field={"pathHash", "parentPathHash", "identifier", "index", "contentObjectProxy", "removed", "dimensionHash", "hidden", "hiddenBeforeDateTime", "hiddenAfterDateTime", "hiddenInIndex", "nodeType", "properties"})
     */
    protected $lastModificationDateTime;

This change adjusts that and now uses the lastPublicationDate with a fallbackt to creationDate as sole source of truth for Zombie detection.