(Refiles to correct attribution in par 11) By Iryna Nazarchuk SERHIIVKA, Ukraine (Reuters) -Russia flattened part of an apartment building while residents slept on Friday in missile attacks near Ukraine’s port of Odesa that authorities said killed at least 21 people, hours after Russian troops abandoned the Black Sea outpost of Snake Island. Residents in the resort village of Serhiivka helped workers pick through the rubble of the nine-storey apartment block, a section of which had been destroyed in the early-morning strike. Walls and windows of a neighbouring 14-storey apartment block were da…