Federal investigators in Manhattan executed a search warrant on Wednesday at the Upper East Side apartment of Rudolph W. Giuliani, the former New York City mayor who became President Donald J. Trump’s personal lawyer, stepping up a criminal investigation into Mr. Giuliani’s dealings in Ukraine, three people with knowledge of the matter said. One of the people said the investigators had seized Mr. Giuliani’s electronic devices. Executing a search warrant is an extraordinary move for prosecutors to take against a lawyer, let alone a lawyer for a former president, and it marks a major turning point in the long-running investigation...