fix: make Request::getEnv() deprecated

[kenjis]

Description
This method does not work from the beginning, and it was intentional.

    public function index(): string
    {
        dd($this->request->getEnv('CI_ENVIRONMENT'));
    }

$this->request->getEnv(...) null

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