By Simon Lewis MEXICO CITY (Reuters) -The United States and Mexico on Friday discussed a new joint security plan to help fight drug cartels as U.S. Secretary of State Antony Blinken met with Mexico's president on a visit aimed to patching up frayed ties. The Biden administration is increasingly reliant on its southern neighbor to stem the flow of Latin American migrants heading to the United States. Overhauling an earlier security plan, known as the Merida Initiative, is expected to take many months as the two sides work out how best to work together following a period of strained cooperation....