Restore checkout header which was reverted in 500c46f

Author: mikejolley

templates/parts/checkout-header.html

@@ -1,9 +1,9 @@
-<!-- wp:group {"layout":{"type":"constrained"}} -->
-<div class="wp-block-group">
+<!-- wp:group {"tagName":"header","area":"header","metadata":{"name":"Checkout Header"},"layout":{"type":"constrained"}} -->
+<header class="wp-block-group">
 	<!-- wp:group {"align":"wide","style":{"spacing":{"padding":{"bottom":"var:preset|spacing|40"}}},"layout":{"type":"flex","justifyContent":"space-between"}} -->
 	<div class="wp-block-group alignwide" style="padding-bottom:var(--wp--preset--spacing--40)">
 		<!-- wp:site-title {"level":0} /-->
 	</div>
 	<!-- /wp:group -->
-</div>
+</header>
 <!-- /wp:group -->