Fixes #68764
Alternatives to #68735

What?

This PR changes the schema for the ariaLabel block support to save its value in the comment delimiter.

That is, the block HTML will be updated to look like this:

<!-- From... -->
<!-- wp:group {"layout":{"type":"constrained"}} -->
<div aria-label="my-label" class="wp-block-group"></div>
<!-- /wp:group -->
<!-- To... -->
<!-- wp:group {"layout":{"type":"constrained"},"ariaLabel":"my-label"} -->
<div aria-label="my-label" class="wp-block-group"></div>
<!-- /wp:group -->

Why?

The current schema references the aria-label attribute if it exists in the HTML markup. This works fine for the Group block, whose HTML markup is preserved, but the value cannot be referenced in the Navigation block, whose HTML markup is not preserved.

As a result, as reported in #68719, when you update a template that contains a Navigation block with the aria-label attribute, the aria-label value disappears.

In Twenty Twenty-Five and Twenty Twenty-Four, aria-label is applied to some Navigation blocks. I believe that the disappearance of this value is a issue from an accessibility perspective.

How?

Remove the source, attribute, and selector fields from the schema definition. This will break Group blocks that already support ariaLabel, so something needs to be done about it.

I noticed that there is already a fix for block validation regarding custom class names (fixCustomClassname() function). I used this process to automatically migrate the attributes via the fixAriaLabel() function so that block validation does not fail.

To solve this problem, I considered three approaches (See #68764). I believe this approach is the best.

Testing Instructions

Group Block

• Open the code editor and enter HTML like below:

  <!-- wp:group {"layout":{"type":"constrained"}} -->
  <div aria-label="my-label" class="wp-block-group"></div>
  <!-- /wp:group -->

• Save the post and refresh your browser. The block should not break.
• Make a change to the post content.
• Open the code editor. The HTML should be updated as below:

  <!-- wp:group {"layout":{"type":"constrained"},"ariaLabel":"my-label"} -->
  <div aria-label="my-label" class="wp-block-group"></div>
  <!-- /wp:group -->

• Save the post and refresh your browser. The block should not break.

Navigation Block

• Activate the Twenty Twenty-Five theme.
• After resetting the Footer template part, open the editor canvas. This template part contains Navigation blocks with aria-labels About, Privacy, and Social.
• Make changes to this template part and save it.
• Open the code editor. The three aria-label values ​​should not have disappeared.
• Open the site. The three aria-label values ​​should not have disappeared.