HTML API: Set $this->html to protected to support subclassing

When the HTML API was introduced a number of fields were switched from private visibility to protected so that Gutenberg and other systems could more easily enhance the behaviors through subclassing. The $this->html property was overlooked but important for systems using the Tag Processor to stich HTML, specifically performing operations on innerHTML and innerText.

Follow-up [55203].
Props dmsnell.
See #57575.



git-svn-id: https://develop.svn.wordpress.org/trunk@55402 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -274,7 +274,7 @@ class WP_HTML_Tag_Processor {
 	 * @since 6.2.0
 	 * @var string
 	 */
-	private $html;
+	protected $html;
 
 	/**
 	 * The last query passed to next_tag().