AMSTERDAM (Reuters) - Dutch insurer Aegon on Monday said it will gradually reduce holdings in companies generating revenue from coal-fired power plants, or coal mining, to support the transition towards a low-carbon economy.Aegon said it already excludes companies that derive more than 30% of sales from the exploration, mining, and refining of thermal coal.It will scale back those investments over the next decade. "As of 2020, a declining revenue threshold has been introduced, which will be..