KUALA LUMPUR, June 20 (Reuters) - Credit agency RAM Ratings said state-backed mortgage lender Cagamas Bhd CAGMAS.UL will issue 20 billion ringgit ($6.22 billion) in commercial paper and sukuk to refinance existing debt.The issue will refinance existing debt of 20 billion ringgit expiring June 25, RAM said in a ratings filing with the central bank late on Thursday, assigning Cagamas&aposs new progr