By Natalia Zinets and Conor Humphries KYIV/POPASNA, Ukraine (Reuters) -The governor of Ukraine’s eastern Luhansk province, now almost completely under Russian control, said on Friday that Kyiv’s forces might have to retreat from their final pocket of resistance there to avoid being captured. Serhiy Gaidai said in a Telegram post that Russian troops had entered Sievierodonetsk, the biggest city held by Ukraine in the eastern Donbas region, where Moscow’s biggest advance for weeks has shifted the momentum of the three-month-old war. Russia has been trying to trap Ukrainian forces in Sievierodone…