Fix content loss in Group block

[Tug]

Fixes #1206

Gutenberg side PR: WordPress/gutenberg#14443

To test:

• Boot the demo app with the following value in initial-html.js

<!-- wp:group -->
<div class="wp-block-group"><div class="wp-block-group__inner-container">
<!-- wp:paragraph -->
<p>hello world</p>
<!-- /wp:paragraph -->
</div></div>
<!-- /wp:group -->

• switch to html and notice the value is preserved.
• Try the repro step on Content loss in Group block #1206 on WPiOS and WPAndroid to make sure no content is loss.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[hypest]

LGTM!

Small note, main WP apps cannot be used directly against this PR since there are some bridge changes (merged in a different PR) that haven't yet been adapted. I manually made the adaptation and verified this PR works OK!

Also, good job helping out on the Gutenberg side PR that @mcsf started!

Feel free to update this PR and merge when the Gutenberg side PR gets merged 👍