Plugin: Deprecate gutenberg_can_edit_* functions

[aduth]

Related: #11015

This pull request seeks to deprecate the gutenberg_can_edit_post_type and gutenberg_can_edit_post functions in favor of the core-equivalent use_block_editor_for_post_type and use_block_editor_for_post functions.

See:

• https://developer.wordpress.org/reference/functions/use_block_editor_for_post_type/
• https://developer.wordpress.org/reference/functions/use_block_editor_for_post/

Testing instructions:

Verify that one can edit a Gutenberg post if and only if the post type and user are capable of editing (e.g. show_in_rest, supports editor, etc). The logic is not identical in core, and this will continue to be aligned until eventually Gutenberg merely extends the edit-form-blocks.php interface.

The core implementations also use the meta-box-loader query parameter for meta boxes, which is not the same as what's used in Gutenberg. However, this is already slated for removal as of #13449.

[jorgefilipecosta]

LGTM 👍
I verified that for this CPT:

function create_product_type() {
    register_post_type( 'acme_product',
        array(
	    'label'  => 'Product',
            'labels' => array(
                'name' => __( 'Products' ),
                'singular_name' => __( 'Product' )
            ),
            'public' => true,
            'show_in_rest' => true,
            'supports' => array( 'title' ),
        )
    );
}
add_action( 'init', 'create_product_type' );

The editor is not loaded, and classic editor opens with just the title field (as before).

If we replace title with editor the editor loads without the option to choose the title.

If we remove the supports array the editor loads normally if we set show_in_rest to false the classic editor starts being used instead.

I also did some grepping and I verified that there are no usages of the deprecated functions.