feat(radio-group): use gray tokens

Author: thetaPC

core/src/components/radio-group/radio-group.native.scss

@@ -20,5 +20,5 @@
 }
 
 .radio-group-top .helper-text {
-  color: $text-color-step-300;
+  color: var(--ion-color-gray-700);
 }