swap out border that is utilizing secondary gray to use

border-color variable

Closes: CNVS-22909

Test plan:
- using old ui navigate to the courses dropdown
- the border around the courses dropdown on hover should now be
  using #e6e6e6 for the top, left & right border

Change-Id: I77f0bde601681db10b3b795f77507a10de32b085
Reviewed-on: https://gerrit.instructure.com/62411
Reviewed-by: Jennifer Stern <[email protected]>
Product-Review: Jennifer Stern <[email protected]>
Tested-by: Jenkins
QA-Review: Myller de Araujo <[email protected]>

app/stylesheets/base/_#header.scss

@@ -191,9 +191,9 @@ $masquerade-link-color: $ic-color-alert;
       text-shadow: none;
       @if $use_new_styles == false {
         color: $course-dropdown-text-color;
-        border-top-color: darken($ic-brand-secondary, 12%);
-        border-left-color: darken($ic-brand-secondary, 12%);
-        border-right-color: darken($ic-brand-secondary, 12%);
+        border-top-color: $ic-border-color;
+        border-left-color: $ic-border-color;
+        border-right-color: $ic-border-color;
         background-color: $courses-dropdown-menu-bg;
       }
     }