Adjust media-text attributes to default stacked on mobile to true

[chipsnyder]

Fixes WordPress/gutenberg#26039

Related PR
gutenberg WordPress/gutenberg#26041

Description

This aligns the deprecated baseAttributes value for isStackedOnMobile to default to false matching the default value for the updated settings.

How has this been tested?

1. In the demo app set the initial HTML content to:

export default `
<!-- wp:media-text {"align":"full","mediaId":696,"mediaType":"image","imageFill":true} -->
<div class="wp-block-media-text alignfull is-stacked-on-mobile is-image-fill"><figure class="wp-block-media-text__media" style="background-image:url('https://a8ctm1.files.wordpress.com/2020/01/dane-deaner-lgjoc2cmff4-unsplash.jpg?w=683');background-position:50% 50%;"><img src="https://a8ctm1.files.wordpress.com/2020/01/dane-deaner-lgjoc2cmff4-unsplash.jpg?w=683" alt="" class="wp-image-696" /></figure>
<div class="wp-block-media-text__content">
<!-- wp:paragraph {"className":"has-large-font-size"} -->
<p class="has-large-font-size"></p>
<!-- /wp:paragraph -->
</div></div>
<!-- /wp:media-text -->

2. Run the demo app
3. Expect the Media-text block to be stacked

Before	After

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

Wanna run full suite of Android and iOS UI tests? Click here and 'Approve' CI job!

[enejb]

It needs a rebase but other then that I think this is ready to

[enejb]

Thanks for the changes!