docs: add missing composer.json in Mandatory File Changes

[kenjis]

Description

• If a user installed CI4 manually, and installed a Composer package,
  Composer package Kint 4.x is installed and used.
  And it is not compatible with the updated Config file for Kint 5.x.
• ComposerScripts::postUpdate will remove Kint files in system/ThirdParty/
  if we remove "kint-php/kint" in composer.json and run composer update.

See codeigniter4/framework@f83c211#diff-d2ab9925cad7eac58e0ff4cc0d251a937ecf49e4b6bf57f8b95aab76648a9d34

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• [] Conforms to style guide

[kenjis]

Removing "kint-php/kint" is not enough.
Because after removing it if we run composer update,
"post-update-cmd" will remove Kint in system/ThirdParty/.

--- a/composer.json
+++ b/composer.json
@@ -9,7 +9,6 @@
         "ext-intl": "*",
         "ext-json": "*",
         "ext-mbstring": "*",
-        "kint-php/kint": "^4.2",
         "laminas/laminas-escaper": "^2.9",
         "psr/log": "^1.1"
     },
@@ -51,9 +50,6 @@
         ]
     },
     "scripts": {
-        "post-update-cmd": [
-            "CodeIgniter\\ComposerScripts::postUpdate"
-        ],
         "test": "phpunit"
     },
     "support": {