Fix: menu-header.php uses relative file_exists() causing wrong submenu URL when PHP CWD is not wp-admin/

[westonruter]

This PR is not tested. I simply applied the suggestion from the reporter. This PR is for testers to validate.

Trac ticket: https://core.trac.wordpress.org/ticket/64721

Use of AI Tools

n/a

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[github-actions]

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