AvatarStack: Prevent border styles from applying to non-avatar components (#7259)

Co-authored-by: TylerJDev <[email protected]>

Author: TylerJDev

.changeset/bitter-cougars-deny.md

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+---
+'@primer/react': patch
+---
+
+AvatarStack: Border was incorrectly applying to elements that were not `Avatar` children.

.changeset/evil-zebras-train.md

@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+AvatarStack: The square prop was not applying to individual `Avatar` components.