Mortgage rates moved slightly higher this week but have remained relatively steady in recent weeks, mortgage buyer Freddie Mac said on Thursday.Freddie Mac's latest Primary Mortgage Market Survey, released Thursday, showed the average interest rate on the benchmark 30-year fixed mortgage rose to 6.49%, up from last week's reading of 6.43%.The average rate on a 30-year fixed-rate mortgage was 6.72% a year ago.HOUSING AFFORDABILITY TO IMPROVE AS H…