The ornamental design for a key, as shown and described.
FIG. 1 is a front view of a key showing my new design;
FIG. 2 is a left side view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a rear perspective view thereof; and,
FIG. 6 is a front perspective view thereof.
The broken lines environment indicates the disclaimed portions of the design. The end of the shading on the surface indicates the boundary of the claimed design. None of the broken lines form part of the claimed design.