Coding Standards: Rename the $object variable to $attachment in several files.

This brings some consistency with a similar fragment in `Custom_Image_Heade::step_2_manage_upload()`, `WP_Site_Icon::insert_attachment()`, `media_handle_upload()`, and clarifies the type of the data.

Follow-up to [52946], [53137].

See #55327, #54728.

git-svn-id: https://develop.svn.wordpress.org/trunk@53183 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-custom-image-header.php

@@ -973,7 +973,7 @@ endif;
 		$file     = $file['file'];
 		$filename = wp_basename( $file );
 
-		// Construct the array with attachment object data.
+		// Construct the attachment array.
 		$attachment = array(
 			'post_title'     => $filename,
 			'post_content'   => $url,