Add box-shadow back to the AvatarStack avatars (#3723)

Co-authored-by: jonrohan <54012+jonrohan@users.noreply.github.com>

Author: jonrohan

.changeset/warm-bottles-smell.md

@@ -0,0 +1,5 @@
+---
+"@primer/view-components": patch
+---
+
+Add box-shadow back to the AvatarStack avatars

.playwright/screenshots/snapshots.test.ts-snapshots/primer/beta/avatar_stack/avatar_2/default.png

@@ -40,7 +40,6 @@
     /* stylelint-disable-next-line primer/colors */
     border-right: var(--borderWidth-thin) solid var(--bgColor-default);
     border-radius: var(--borderRadius-small);
-    box-shadow: none;
     transition: margin 0.1s ease-in-out;
 
     &:first-child {