We claim the ornamental design for a keyboard key, as shown and described.
FIG. 1 is a front perspective view of a keyboard key;
FIG. 2 is a front elevational view of the keyboard key;
FIG. 3 is a left elevational view thereof;
FIG. 4 is a right elevational view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken border lines shown in FIGS. 1 and 2 immediately surrounding the keys represent the bounds of the claimed design, while all other lines are directed to environment and form no part of the claimed design.