Revert "style: update free course notice text color to teal-600"

Author: rohitpaulk

app/components/course-overview-page/notices/free-course-notice.hbs

@@ -15,7 +15,7 @@
           {{date-format @course.isFreeUntil format="d MMMM yyyy"}}.
         {{/if}}
       </h3>
-      <p class="text-teal-600 text-sm mb-4">
+      <p class="text-teal-800 text-sm mb-4">
         The core challenge experience is completely free
         {{#if @course.isFreeThisMonth}}
           this month.