HTML API: Lower-case HTML tag names in get_qualified_tag_name().

Since this method is meant for printing and display, a more expected
return value would be the lower-case variant of a given HTML tag name.

This patch changes the behavior accordingly.

Author: dmsnell

src/wp-includes/html-api/class-wp-html-tag-processor.php

@@ -2840,7 +2840,7 @@ public function get_qualified_tag_name(): ?string {
 		}
 
 		if ( 'html' === $this->get_namespace() ) {
-			return $tag_name;
+			return strtolower( $tag_name );
 		}
 
 		$lower_tag_name = strtolower( $tag_name );