feat: "system/bootstrap.php" shows upgrade error message

Author: kenjis

system/bootstrap.php

@@ -13,8 +13,7 @@
 
 /**
  * ---------------------------------------------------------------
- *
- * @deprecated 4.5.0 This file is no longer used. Moved to Boot.php.
+ * This file cannot be used. The code has moved to Boot.php.
  * ---------------------------------------------------------------
  */
 
@@ -24,6 +23,15 @@
 use Config\Paths;
 use Config\Services;
 
+header('HTTP/1.1 503 Service Unavailable.', true, 503);
+
+$message = 'This "system/bootstrap.php" is no longer used. If you are seeing this error message,
+the upgrade is not complete. Please refer to the upgrade guide and complete the upgrade.
+See https://codeigniter4.github.io/userguide/installation/upgrade_450.html' . PHP_EOL;
+echo $message;
+
+exit(1);
+
 /*
  * ---------------------------------------------------------------
  * SETUP OUR PATH CONSTANTS
@@ -37,7 +45,7 @@
 /** @var Paths $paths */
 
 // The path to the application directory.
-if (! defined('APPPATH')) {
+if (! defined('APPPATH')) { // @phpstan-ignore-line
     define('APPPATH', realpath(rtrim($paths->appDirectory, '\\/ ')) . DIRECTORY_SEPARATOR);
 }
 

user_guide_src/source/changelogs/v4.5.0.rst

@@ -251,6 +251,8 @@ Others
 - **BaseModel:** The ``getIdValue()`` method has been changed to ``abstract``.
 - **Routing:** The :ref:`404-override` feature does change the Response status
   code to 404 by default. See :ref:`Upgrading Guide <upgrade-450-404-override>`.
+- **system/bootstrap.php:** This file cannot be used. The code has been moved to
+  the new ``CodeIgniter\Boot`` class.
 
 Interface Changes
 =================
@@ -471,8 +473,6 @@ Deprecations
       been moved to the ``Autoloader``.
     - The ``configureKint()`` method has been deprecated. No longer used. It has
       been moved to the ``Autoloader``.
-- **system/bootstrap.php:** This file has been deprecated. No longer used.
-  The code has been moved to the new ``CodeIgniter\Boot`` class.
 - **Response:** The constructor parameter ``$config`` has been deprecated. No
   longer used.
 - **Filters:**