fix: cast $expired as int in wp_validate_auth_cookie()

[justlevine]

Trac ticket: https://core.trac.wordpress.org/ticket/52217

This PR fixes an issue in wp_validate_auth_cookie() where the (string) $expired is used both in a comparison and addition with int values in .

The issue is fixed by casting $expired to an int when it is defined.

While this issue was surfaced via PHPStan in #7619 (trac: https://core.trac.wordpress.org/ticket/61175 ), it can be remediated independently of that ticket.

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