Page Attributes: Fix: Parent-Dropdown missing for custom post-type.

[jorgefilipecosta]

Fix: #7926
Regressed in: #6657

We were allowing unbound requests for pages because pages have a parent picker UI that needed unbound requests. Pages were not the only CPT's that allow choosing a parent post. This commit makes sure we allow unbound requests for all hierarchical post types.

Here we try to keep the same logic added in #6657 but we apply it to all Hierarchical CPT's and not just pages.

A minor correction to the documentation was also applied, the parameter $post_type of the function gutenberg_filter_post_collection_parameters is a (WP_Post_Type) oject not a string. https://developer.wordpress.org/reference/hooks/rest_this-post_type_collection_params/

How has this been tested?

I added a hierarchical CPT and verified the Parent page picker appeared in the page attributes settings:

 function myplugin_register_hierarchical_book_type() {
    $args = array(
        'public' => true,
        'label'  => 'Books Hierarchical',
        'show_in_rest' => true,
        'hierarchical'       => true,
        'supports'       => array( 'page-attributes' ),
    );
    register_post_type( 'book_lock', $args );
}
add_action( 'init', 'myplugin_register_hierarchical_book_type' );