The ornamental design for a breastplate, as shown and described.
FIG. 1 is a front view of a breast plate showing my new design; and,
FIG. 2 is a back view thereof.
All surfaces not shown form no part of the claimed design.
The broken lines showing the flag and the “Made in the USA,” label form no part of the claimed design. All other broken lines represent thread stitching that form no part of the claimed design.