The average long-term U.S. mortgage rate rose this week, pushing up borrowing costs for prospective homebuyers in the midst of the spring homebuying season. The benchmark 30-year fixed rate mortgage rate rose to 6.3% from 6.23% last week, mortgage buyer Freddie Mac said Thursday. That’s still down from one year ago, when the rate averaged 6.76%. The increase ends a three-week slide, bringing the average rate back to where it was two weeks ago. Borrowing costs on 15-year fixed-rate mortgages,