docs: add note for PHP and CI4 versions

Author: kenjis

user_guide_src/source/intro/requirements.rst

@@ -22,6 +22,13 @@ PHP and Required Extensions
     - If you are still using PHP 7.4 or 8.0, you should upgrade immediately.
     - The end of life date for PHP 8.1 will be December 31, 2025.
 
+.. note::
+    - PHP 8.4 requires CodeIgniter 4.6.0 or later.
+    - PHP 8.3 requires CodeIgniter 4.4.4 or later.
+    - PHP 8.2 requires CodeIgniter 4.2.11 or later.
+    - PHP 8.1 requires CodeIgniter 4.1.6 or later.
+    - **Note that we only maintain the latest version.**
+
 ***********************
 Optional PHP Extensions
 ***********************