Banner: Add flush prop for use inside dialogs, sidebars and other confined spaces (#7250)

Co-authored-by: Marie Lucca <40550942+francinelucca@users.noreply.github.com>

Author: siddharthkp

.changeset/flush-banner.md

@@ -0,0 +1,5 @@
+---
+"@primer/react": minor
+---
+
+Banner: Add `flush` prop for use within confined spaces, such as dialogs, tables, cards, or boxes where available space is limited.