chore: move preload.php

[kenjis]

Description
Follow-up #6088

codeigniter4/framework also should have this file.

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

[samsonasik]

If preload.php is on framework part, which in vendor/ location already, the documentation usage may need update:

CodeIgniter4/admin/starter/preload.php

Line 51 in 197e996

'include' => __DIR__ . '/vendor/codeigniter4/framework/system',

CodeIgniter4/admin/starter/preload.php

Line 19 in 197e996

* 1. Set preload::$paths.

Or probably better change to:

'include' => __DIR__ . '/system',

on inside framework

[kenjis]

Updated the comment.

The preload.php should be located in the project root folder.
Devs should customize it.

If it is only in appstarter,

1. devs who do not use appstarter can't get it easily.
2. when it is updated, devs can't update it easily.

[samsonasik]

Looks good 👍

[samsonasik]

Thank you @kenjis