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/boards/shields/hummingbird/hummingbird.keymap

@@ -66,7 +66,7 @@
         };
 
         nav_layer {
-            label = "Nav";
+            display-name = "Nav";
             bindings = <
                 &trans     &trans     &trans      &trans          &trans       &trans           &kp HOME       &kp UARW &kp PG_UP  &trans
                 &trans     &trans     &trans      &trans          &trans       &trans           &kp LARW       &kp DARW &kp RARW   &trans
@@ -76,7 +76,7 @@
         };
 
         num_layer {
-            label = "Num";
+            display-name = "Num";
             bindings = <
                 &kp LBKT   &kp N7     &kp N8      &kp N9          &kp RBKT      &trans          &trans         &trans      &trans     &trans
                 &kp SEMI   &kp N4     &kp N5      &kp N6          &kp EQUAL     &trans          &trans         &trans      &trans     &trans
@@ -86,7 +86,7 @@
         };
 
         sym_layer {
-            label = "Sym";
+            display-name = "Sym";
             bindings = <
                 &kp LBRC   &kp LS(N7) &kp LS(N8)  &kp LS(N9)      &kp RBRC      &trans          &trans         &trans      &trans     &trans
                 &kp COLON  &kp LS(N4) &kp LS(N5)  &kp LS(N6)      &kp PLUS      &trans          &trans         &trans      &trans     &trans