(Reuters) – Ukraine’s President Volodymyr Zelenskiy sacked the head of the country’s domestic security service and state prosecutor, citing hundreds of cases of alleged treason and collaboration with Russia, as Moscow appeared set to step up military operations. FIGHTING * Russia’s defence ministry said its aircraft had shot down a Ukrainian MI-17 helicopter near the eastern town of Sloviansk and an SU-25 aircraft in the Kharkiv region. * The Russian army also said its long-range air-based missiles had destroyed a depot in an industrial zone of Odesa, southern Ukraine, that stored Harpoon anti…