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.

git-svn-id: https://develop.svn.wordpress.org/trunk@49993 602fd350-edb4-49c9-b593-d223f7449a82

src/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">

src/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