docs: remove sending PR to FriendsOfPHP/security-advisories

[kenjis]

Description
composer audit uses GitHub Security Advisory Database.
https://github.com/advisories?query=codeigniter4
So now we don't need to send PR to FriendsOfPHP/security-advisories.

$ php spark | head -2

CodeIgniter v4.3.4 Command Line Tool - Server Time: 2023-05-23 23:59:12 UTC+00:00

$ composer audit
Info from https://repo.packagist.org: #StandWithUkraine
Found 1 security vulnerability advisory affecting 1 package:
+-------------------+----------------------------------------------------------------------------------+
| Package           | codeigniter4/framework                                                           |
| CVE               | CVE-2023-32692                                                                   |
| Title             | Remote Code Execution Vulnerability in Validation Placeholders in CodeIgniter4   |
| URL               | https://github.com/advisories/GHSA-m6m8-6gq8-c9fj                                |
| Affected versions | <4.3.5                                                                           |
| Reported at       | 2023-05-22T19:49:11+00:00                                                        |
+-------------------+----------------------------------------------------------------------------------+

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