KHARKIV, Ukraine (Reuters) -Twin Russian missile strikes on central Kharkiv on Saturday injured at least 21 people, Ukrainian officials said. The Kharkiv regional chief prosecutor said two boys aged 14 and 16 and a security advisor for a team of German journalists were among those injured. The missiles came from the direction of Belgorod, Russia, he added. Russian officials said a Ukrainian attack on the provincial capital of Belgorod hours earlier killed 18 people and injured 111. That came a day after Russia’s biggest air assault of the war, in which Ukrainian officials said at least 40 civi…