feat(checkbox): use gray tokens

Author: thetaPC

core/src/components/checkbox/checkbox.native.scss

@@ -82,7 +82,7 @@
 }
 
 .checkbox-bottom .helper-text {
-  color: $text-color-step-300;
+  color: var(--ion-color-gray-700);
 }
 
 // Label Placement - Start