By Jonathan Landay NEAR KHERSON FRONTLINE, Ukraine (Reuters) -Russia on Tuesday took its case to the U.N. Security Council that Ukraine is preparing to use a “dirty bomb” on its own territory, an assertion dismissed by Western and Ukrainian officials as misinformation and a pretext for intensifying the war. Moscow sent a letter detailing the allegations to the United Nations on Monday, and Russia raised the issue at a closed meeting with the Security Council. “We’re quite satisfied because we raised the awareness,” Russia’s Deputy U.N. Ambassador Dmitry Polyanskiy told reporters. “I don’t mind…