The ornamental design for a milkfrother, as shown and described.
FIG. 1 is a left front perspective view showing the bottom, front and left side of the milk frother of our new design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof; and
FIG. 4 is a side view thereof, all sides being identical hereto; and,
FIG. 5 is a left front perspective view similar to FIG. 1, exploded to show portions of the milk frother separated from each other along a common axis.