Fix: Embed: Block transformed to a paragraph keeps classes

[petitphp]

What?

Fix #39616. Remove internal css classes, added by the embed block when using some variations like youtube, when transforming the block to another type.

Why?

Related to #38964, the feature allow user-added custom CSS classes to be carried over when transforming blocks. Unfortunately, the Embed block stores its aspect ratio classes as custom CSS classes and they are also kept during transform.

How?

Reusing the same filter as the custom classname support blocks.switchToBlockType.transformedBlock, we apply the removeAspectRatioClasses to the list of classname when transforming an embed block to another type. This allow only remove the aspect ration classes and the keep user-added ones.

Testing Instructions (Base on the instructions from the original issue)

1. Embed a youtube embed block into any block editor post.
2. Check the block "Additional CSS classes" field. It should contain the values wp-embed-aspect-16-9 and wp-has-aspect-ratio
3. Transform the youtube block to a paragraph (ie, paragraph with a linked text)
4. Check again the block "Additional CSS classes" field. It shouldn't contain the classes wp-embed-aspect-16-9 and wp-has-aspect-ratio anymore

[github-actions]

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Co-authored-by: petitphp <[email protected]>
Co-authored-by: ZebulanStanphill <[email protected]>
Co-authored-by: kathrynwp <[email protected]>
Co-authored-by: ndiego <[email protected]>
Co-authored-by: jorgefilipecosta <[email protected]>
Co-authored-by: ts-mz <[email protected]>

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[Mamaduka]

Closing in favor of #68721.