Cameroon’s government has banned the media from discussing the health of President Paul Biya, Africa’s second-longest-ruling leader, amid rumours the 91-year-old is gravely ill, according to a document obtained by AFP.Biya, the world’s oldest leader, has not made a public appearance since early September, fuelling a swirl of rumours online that the veteran president’s health is failing.”The head of state is the principal institution of the republic, and discussions on his condition are a matter of national security,” said the document addressed to regional governors, which was dated…