Hooks: add_filter() used for admin_print_styles action

[apermo]

Problem

In wp-admin/includes/admin-filters.php at line 56, add_filter() is used to register a callback on the admin_print_styles hook, but this hook is fired with do_action() — making it an action, not a filter.

add_filter( 'admin_print_styles', 'wp_resource_hints', 1 );

Where the hook fires

• src/wp-admin/admin-header.php:137 — do_action( 'admin_print_styles' )
• src/wp-admin/includes/template.php:2165
• src/wp-admin/includes/media.php:587
• src/wp-includes/class-wp-customize-widgets.php:674

Fix

Change add_filter() to add_action(). Functionally equivalent (since add_action() calls add_filter() internally), but semantically correct.

Related to https://core.trac.wordpress.org/ticket/64224