BOSTON—A former Massachusetts mayor first elected at the age of 23 was convicted on Friday of defrauding investors in a smartphone app company and extorting money from local marijuana businesses. Jurors found former Fall River Mayor Jasiel Correia guilty on most counts he faced including wire fraud, filing false tax returns, and extortion, in what was the first high-profile federal trial in Boston since the COVID-19 pandemic began. He faces sentencing on Sept. 20. His lawyer, Kevin Reddington, told reporters that he was “disappointed in the verdict, and we do plan to file a vigorous appeal.” He had argued at the trial that his client had “no intent to defraud.” Prosecutors said that before being elected, Correia betrayed investors who sank $363,690 into SnoOwl, a company developing an app to connect local businesses with consumers. Assistant U.S. Attorney Zach Hafer told jurors that SnoOwl was a “con,” and Correia was …