refactor: do not use future-deprecated DATE_RFC7231 constant

[paulbalandan]

Description
It will be deprecated in PHP 8.5: https://wiki.php.net/rfc/deprecations_php_8_5#deprecate_date_rfc7231_and_datetimeinterfacerfc7231

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

[michalsn]

While the constant is deprecated, I don't believe we should ditch the GMT format here, as the expire value for the cookie header should use this date format.

[paulbalandan]

Yes, that is true. But we are just relying this time on the T TZ format instead of the hardcoded GMT timezone (which was the reason why the constant was deprecated). Also, in Cookie::getExpiresString() we already use the gmdate() call ensuring the resulting date string is always in GMT.

[michalsn]

With gmdate() that makes sense. Thanks!