Fix improper reactivation in reimporter, using is_mitigated

[Gby56]

Due to incomplete fix in https://github.com/DefectDojo/django-DefectDojo/pull/6452 Explainer: The initial fix was too narrow in its decision/conditions, only looking at items/findings that had a mitigated timestamp. Instead, is_mitigated should be used, to simply check if the incoming item is already mitigated or not, and if the existing finding is mitigated or not. I have left the timestamp comparison IF it exists on the item.