refactor(keymaps): Rename layer label to "display-name"

Changed the property used to define a layer name for displays from
"label" (which affects other things in Zephyr and is deprecated) to
"display-name". (It cannot be named simply "name", because that has
special meaning in newer versions of the devicetree compiler.)

"label" is still supported as a fallback, so no changes need to be made
to existing keymaps.

app/dts/bindings/zmk,keymap.yaml

@@ -7,12 +7,19 @@ child-binding:
   description: "A layer to be used in a keymap"
 
   properties:
-    label:
+    display-name:
       type: string
       required: false
+      description: The name of this layer to show on displays
     bindings:
       type: phandle-array
       required: true
     sensor-bindings:
       type: phandle-array
       required: false
+
+    label:
+      type: string
+      required: false
+      deprecated: true
+      description: Deprecated. Use "name" instead.