(Reuters) -Britain’s Boris Johnson, one of Ukraine’s staunchest backers, flew to Kyiv on Saturday to pledge tighter sanctions on Russia and offer President Volodymyr Zelenskiy more defensive arms, a move the Ukrainian leader said others should follow. At a meeting shrouded in secrecy until Johnson appeared in the Ukrainian capital, the two leaders cemented the close ties they have nurtured since Russia invaded Ukraine on Feb. 24. The move caps weeks of lobbying by Johnson to meet Zelenskiy. With both men standing at podiums in front of cameras, they praised each other for their cooperation sin…