REST API: Add the missing 'site_icon_url' to the index (#42957)

* REST API: Add the missing 'site_icon_url' to the index
* Add backport note

lib/compat/wordpress-6.1/rest-api.php

@@ -43,3 +43,21 @@ function gutenberg_register_gutenberg_rest_block_patterns() {
 	$block_patterns->register_routes();
 }
 add_action( 'rest_api_init', 'gutenberg_register_gutenberg_rest_block_patterns', 100 );
+
+/**
+ * Exposes the site logo URL through the WordPress REST API.
+ *
+ * This is used for fetching this information when user has no rights
+ * to update settings.
+ *
+ * Note: Backports into wp-includes/rest-api/class-wp-rest-server.php file.
+ *
+ * @param WP_REST_Response $response REST API response.
+ * @return WP_REST_Response $response REST API response.
+ */
+function gutenberg_add_site_icon_url_to_index( WP_REST_Response $response ) {
+	$response->data['site_icon_url'] = get_site_icon_url();
+
+	return $response;
+}
+add_action( 'rest_index', 'gutenberg_add_site_icon_url_to_index' );