Twenty Twenty-One: Add missing HTML comments for </header> tag.

This brings some consistency to closing `</header>` tags in theme templates and ensures they all have an associated HTML comment.

Props freewebmentor, mukesh27.
Fixes #52328.
Built from https://develop.svn.wordpress.org/trunk@49993


git-svn-id: http://core.svn.wordpress.org/trunk@49694 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-content/themes/twentytwentyone/template-parts/content/content-page.php

@@ -17,11 +17,11 @@
 		<header class="entry-header alignwide">
 			<?php get_template_part( 'template-parts/header/entry-header' ); ?>
 			<?php twenty_twenty_one_post_thumbnail(); ?>
-		</header>
+		</header><!-- .entry-header -->
 	<?php elseif ( has_post_thumbnail() ) : ?>
 		<header class="entry-header alignwide">
 			<?php twenty_twenty_one_post_thumbnail(); ?>
-		</header>
+		</header><!-- .entry-header -->
 	<?php endif; ?>
 
 	<div class="entry-content">

wp-content/themes/twentytwentyone/template-parts/content/content-single.php

@@ -16,7 +16,7 @@
 	<header class="entry-header alignwide">
 		<?php the_title( '<h1 class="entry-title">', '</h1>' ); ?>
 		<?php twenty_twenty_one_post_thumbnail(); ?>
-	</header>
+	</header><!-- .entry-header -->
 
 	<div class="entry-content">
 		<?php

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7-alpha-49992';
+$wp_version = '5.7-alpha-49993';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.