Fix border-color on ActionList Group header (#5409)

* fix border color on header

* Create popular-moose-end.md

* test(vrt): update snapshots

* Update packages/react/src/ActionList/Group.tsx

---------

Co-authored-by: langermank <langermank@users.noreply.github.com>

Author: langermank

.changeset/popular-moose-end.md

@@ -0,0 +1,5 @@
+---
+"@primer/react": patch
+---
+
+Fix `border-color` on ActionList Group header

.playwright/snapshots/components/ActionList.test.ts-snapshots/ActionList-Group-With-Filled-Title-Old-Api-dark-high-contrast-linux.png

@@ -164,7 +164,7 @@ export const GroupHeading: React.FC<React.PropsWithChildren<ActionListGroupHeadi
       marginBottom: 2,
       borderTop: '1px solid',
       borderBottom: '1px solid',
-      borderColor: 'neutral.muted',
+      borderColor: 'var(--borderColor-muted)',
     }),
 
     [`.ActionListGroupHeading`]: {