BRUSSELS — European Union officials have tumbled into a cyber crisis after attacks hit its digital systems and officials’ phones. Cyber experts are probing to see just how deep the rabbit hole goes. The EU executive has told some of its most senior officials to shut down a group on messaging app Signal over fears it was a hacking target, POLITICO first reported on Thursday. The move comes as the EU faced a string of attacks in the last few months that led to breaches of its cloud infrastructure