[Keyboard] Rename ymdk_np21 to np21, move to ymdk vendor folder (#15641)

keyboards/ymdk/np21/np21.c

@@ -16,4 +16,4 @@ You should have received a copy of the GNU General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#include "ymdk_np21.h"
+#include "np21.h"

keyboards/ymdk/np21/readme.md

@@ -10,11 +10,11 @@ ps2avrGB based number-pad sold fully assembled by YMDK on Aliexpress.
 
 Make example for this keyboard (after setting up your build environment):
 
-    make ymdk_np21:default
+    make ymdk/np21:default
 
 Flashing example for this keyboard ([after setting up the bootloadHID flashing environment](https://docs.qmk.fm/#/flashing_bootloadhid))
 
-    make ymdk_np21:default:flash
+    make ymdk/np21:default:flash
 
 **Reset Key**: Hold down the 'Top Left Key' (USB on top) while plugging in the keyboard.