The ornamental design for a serverunitplenum, as shown and described.
FIG. 1 is a side view of a server unit with a server unit plenum showing our design;
FIG. 2 is a perspective view thereof;
FIG. 3 is a plan view of the server unit plenum;
FIG. 4 is a side view thereof;
FIG. 5 is a first end view thereof;
FIG. 6 is a second end view thereof; and,
FIG. 7 is a perspective view thereof.
The broken lines in the drawings illustrate portions of the server unit, or the server unit plenum, or environment and form no part of the claimed design.