REST API: Add the missing site_icon_url to the index.

The `site_icon_url` index was supposed to ship with WordPress 5.6, but was [WordPress/gutenberg#22952 never backported to core].

This commit backports the original PR from Gutenberg repository:
* [WordPress/gutenberg#42957 #42957: REST API: Add the missing 'site_icon_url' to the index]

Follow-up to [52080].

Props Mamaduka, bernhard-reiter, TimothyBlynJacobs.
See #56467.
Built from https://develop.svn.wordpress.org/trunk@54083


git-svn-id: http://core.svn.wordpress.org/trunk@53642 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api/class-wp-rest-server.php

@@ -1308,6 +1308,8 @@ protected function add_site_icon_to_index( WP_REST_Response $response ) {
 		$site_icon_id = get_option( 'site_icon', 0 );
 
 		$this->add_image_to_index( $response, $site_icon_id, 'site_icon' );
+
+		$response->data['site_icon_url'] = get_site_icon_url();
 	}
 
 	/**

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-54082';
+$wp_version = '6.1-alpha-54083';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.