The ornamental design for a milkfrother, as shown and described.
FIG. 1 is a perspective view of a milk frother showing my new design;
FIG. 2 is a right side elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a front elevational view thereof; and,
FIG. 5 is a top plan view thereof.
The broken line showing of a coffee maker in FIGS. 1-5 depicts environment and forms no part of the claimed design. All other broken lines depict portions of the milk frother that form no part of the claimed design.