Home prices in Boston are still on the rise, but the rate of change is slowing, according to March figures released Tuesday by the widely followed S&P/Case-Shiller Home Price Index. In Boston, home prices rose at an annual rate of 8.2 percent, their slowest growth in six months. Nationally, prices increased at an annual rate of 12.4 percent. Higher mortgage rates and tight credit requirements have contributed to the slowdown, locally and nationally.Bennie DiNardo can be reached at firstname.lastname@example.org. Follow him on Twitter @bdinardo.