DETROIT — A Waterford Township man convicted of plotting to kidnap Gov. Gretchen Whitmer deserves a “substantial” federal prison sentence to deter rising threats from domestic extremists bent on achieving political change through violence, prosecutors said Thursday. But prosecutors urged U.S. District Judge Robert Jonker to consider Kaleb Franks pleaded guilty and cooperated with the government, testifying against two ringleaders during two trials this year. Advisory guidelines call for as long as 15 1/2 years in prison. Franks, 28, will be sentenced Oct. 6, two months after he helped secure t…