What?

Syncing a change that was made in the core sync PR for grid and column spans.

Using wp_unique_id to generate the child layout classname number means that number will vary depending on how many times wp_unique_id has been used on the page before. This was causing the results of the test added in #61392 to be unpredictable, and also the classname of any given block on a page to be unpredictable, which could affect things like dynamic loading (see #55416).

This PR changes the child layout classname to use wp_unique_prefixed_id instead.

Testing Instructions

Nothing should have visibly changed, but if you

1. Create a post with:
   • an Image block with a custom duotone filter applied (choose the colors yourself, don't use the preset);
   • a Row block with a few children, and set one of the children to "Fill" under Dimensions;
2. Create a custom template for that post that contains only the Post Content block;
3. Save and view on the front end. The classname for the Row child set to Fill should be wp-container-content-1 (in trunk it'll be wp-container-content-2 because the duotone filter also uses wp_unique_id).

Testing Instructions for Keyboard

Screenshots or screencast