Custom attributes for list block are lost when migrating from wordpress 5.5.1 to wordpress 6.1.1

[kmrmohit]

Description

When upgrading from wordpress 5.5.1 to wordpress 6.1.1, the custom attributes defined for list block in wordpress 5 are not retained while migrating the list block.

Step-by-step reproduction instructions

1. Run wordpress 6.1.1
2. Open the code editor and paste the below html created for wp5 list blocks

<!-- wp:list {"someCustomAttribute":["attrVal1","attrVal2]} -->
<ul><li>first</li><li>second</li></ul>
<!-- /wp:list -->

3. Switch to visual editor and let wordpress create the blocks
4. Open the code editor again and check for someCustomAttribute in the wp:list attributes,
   or open the console and execute below command
   wp.data.select('core/editor').getBlocks().attributes
   The code editor in wordpress 6 shows the below html and doesn't have the custom attribute anymore

<!-- wp:list -->
<ul><!-- wp:list-item -->
<li>first</li>
<!-- /wp:list-item -->

<!-- wp:list-item -->
<li>second</li>
<!-- /wp:list-item --></ul>
<!-- /wp:list -->

Screenshots, screen recording, code snippet

No response

Environment info

• Wordpress 6.1.1
• Gutenberg 14.3.1
• Chrome Version 124.0.6367.119 (Official Build) (arm64)
• macOS 14.3.1 (23D60)

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[kmrmohit]

#45987