By Lucia Mutikani WASHINGTON (Reuters) – The U.S. economy notched its strongest growth in nearly four decades in 2021 after the government pumped trillions of dollars in COVID-19 relief, and is seen forging ahead despite headwinds from the pandemic, strained supply chains as well as inflation. A surge in gross domestic product in the fourth quarter as businesses replenished depleted inventories to meet strong demand for goods was the final push. Last year’s robust growth reported by the Commerce Department on Thursday supports the Federal Reserve’s pivot towards raising interest rates in March…