perf: defer instantiation of Validation in Model

[kenjis]

Description
Closes #7935

• defer instantiation of Validation in Model

Memory Usage:

before: 17392
 after:  7216

<?php

namespace App\Controllers;

use CodeIgniter\Model;

class Home extends BaseController
{
    public function index(): string
    {
        $before = memory_get_usage();
        $model = new Model();
        $after = memory_get_usage();

        return $after - $before;
    }
}

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

[MGatner]

Very nice