The ornamental design for booster seat, as shown and described.
FIG. 1 is an isometric view of the booster seat showing our new design;
FIG. 2 is a top view of the booster seat of FIG. 1;
FIG. 3 is a bottom view of the booster seat of FIG. 1;
FIG. 4 is a front view of the booster seat of FIG. 1;
FIG. 5 is a back view of the booster seat of FIG. 1;
FIG. 6 is a right side view of the booster seat of FIG. 1; and,
FIG. 7 is a left side view of the booster seat of FIG. 1.
The break lines in the drawings, if any, indicate indeterminate length and fins only, and do not form part of the claimed design. Broken lines, if any included, in the drawings are for illustrative purposes only and do not form part of the claimed design.