fix: set_cookie() $expire type

[kenjis]

Description
In the docs, the param $expire is "Number of seconds until expiration", and the type is int:
https://www.codeigniter.com/user_guide/helpers/cookie_helper.html#set_cookie

So it should be int, not string.

The param $expires_or_options for setcookie() is also int.
https://www.php.net/manual/en/function.setcookie.php

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