Medicare is wasting more than $8 billion on an experimental program that rewards providers of mediocre health care and is unlikely to produce useful results, federal investigators say in a new report. The report, to be issued Monday by the Government Accountability Office, a nonpartisan investigative arm of Congress, urges the Obama administration to cancel the program, which pays bonuses to health insurance companies caring for millions of Medicare beneficiaries. Administration officials, however, defended the project and said they would not cancel it because it could improve the quality of care for older Americans.