Add shadow to Popover component (#7198)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: lukasoppermann <813754+lukasoppermann@users.noreply.github.com>
Co-authored-by: Lukas Oppermann <lukasoppermann@github.com>

Author: Copilot

.changeset/gentle-peas-kick.md

@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+Popover: add missing shadow