The ornamental design for a serverunit, as shown and described.
FIG. 1 is a front view of a server unit showing our design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a first side view thereof;
FIG. 5 is a second side view thereof, opposite the first side; and,
FIG. 6 is another front view thereof.
The broken lines in the drawings illustrate portions of the server unit, or environment, and form no part of the claimed design. Cross-hatched lines in the drawings illustrate color and form no part of the claimed design.