fix: deprecate video playlist block

[dkoo]

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Changes proposed in this Pull Request:

Deprecates the YouTube Video Playlist block, as this functionality is now a core feature of YouTube itself.

If the block already exists in any site content, the block will continue to render on the front-end, but editor instances will no longer be editable and will display a deprecation warning.

If the block doesn't exist in any site content, it won't be registered and therefore can't be added to any post content.

At a future date, we can consider removing all block assets from this repo.

How to test the changes in this Pull Request:

1. On trunk, add some YouTube video embeds to post content. Make sure to embed using full, unshortened YouTube URLs to embed or they won't be recognized by the Video Playlist block.
2. Add a YouTube Video Playlist block to another post or page. Confirm that it renders a preview in the editor and renders videos on the front-end.
3. Check out this branch. Confirm that the block continues to render without changes on the front-end.
4. Confirm that in the editor, the block shows a deprecation warning both in the block preview and in the sidebar, and that there's no longer UI to edit block attributes. Confirm that the links in the warning message link out to Google help docs explaining how to create/manage and embed YouTube playlists.

5. Remove the YouTube Video Playlist block from the post (and any other posts/pages it might exist in*), save, then refresh the editor. Confirm that the YouTube Video Playlist block can no longer be found in the block inserter.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

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[matticbot]

🎉 This PR is included in version 4.2.3-alpha.1 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

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