By Alessandra Prentice and Natalia Zinets ZAPORIZHZHIA, Ukraine (Reuters) -Scores of Ukrainians remained trapped on Thursday in a Mariupol steel works that has been rocked by heavy explosions as Russian forces fought for control of Ukraine’s last stronghold in the ruined city and the United Nations rushed to evacuate civilians. Ukrainian President Volodymyr Zelenskiy said more civilians had been rescued from Mariupol but Russia was shelling the Azovstal steel plant where an estimated 200 civilians were still in underground bunkers with little food or water. President Vladimir Putin said Russia…