code quality: add explicit : void to method signatures in wp-signup.php, WP_Posts_List_Table, and WP_Screen

[justlevine]

This PR updates the method signatures of several functions and files to enforce a : void return type. Changes are non-breaking and unlikely to disrupt other open PRs.

More specifically:

• wp-signup.php : an example of global functions (cant be overwritten)
• WP_Posts_List_Table : an example of private methods (don't get inherited)
• WP_Screen : an example of methods on a final class (cant be extended)

While surfaced and part of PHPStan efforts (https://core.trac.wordpress.org/ticket/61175 / #7619 ), this PR serves as proof of concept for a larger proposal that would see : void return types implemented across the codebase in a nondisruptive manner while lay the groundwork for safely implementing other return types to existing method signatures.

Trac ticket: https://core.trac.wordpress.org/ticket/63558

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