By Max Hunder and Mari Saito KYIV/KHARKIV, Ukraine (Reuters) -Ukrainian forces endured heavy artillery barrages on Sunday as Russian forces intensified their attacks to capture a city in the eastern region of Luhansk, which President Volodymyr Zelenskiy said was now Moscow’s top target. Constant Russian shelling has destroyed all of the critical infrastructure in Sievierodonetsk, the largest city Ukraine still controls in Luhansk, Zelenskiy said, while pledging to do everything to hold off the advance. “Some 90% of buildings are damaged. More than two-thirds of the city’s housing stock has bee…