Britain’s Prince Harry has sharply attacked the failure of social media companies to challenge hate online, revealing that he warned the chief executive of Twitter ahead of the January 6 Capitol attack that the site was being used to stage political unrest. Harry made the comments Tuesday in an online panel on misinformation in California. He said he made his concerns known via email to Twitter CEO Jack Dorsey the day before the riot in Washington. “Jack and I were emailing each other prior to January 6 where I warned him that his platform was allowing a coup to be…