KABUL, Afghanistan — The Taliban captured two major Afghan cities, the country’s second- and third-largest after Kabul, and a strategic provincial capital on Thursday, further squeezing the embattled government just weeks before the end of the American military mission there. The seizure of Kandahar and Herat marks the biggest prizes yet for the Taliban, who have taken 12 of Afghanistan’s 34 provincial capitals as part of a weeklong blitz.