chore: rename notification banners to announcement banners (#13419)

coderd/database/migrations/000213_announcement_banners.down.sql

@@ -0,0 +1,3 @@
+update site_configs SET
+	key = 'notification_banners'
+	where key = 'announcement_banners';

coderd/database/migrations/000213_announcement_banners.up.sql

@@ -0,0 +1,3 @@
+update site_configs SET
+	key = 'announcement_banners'
+	where key = 'notification_banners';