Fix: YouTube Shorts embeds do not resize correctly on mobile

[wprashed]

YouTube Shorts embeds are displayed with an incorrect aspect ratio on mobile devices, causing the iframe to overflow its container and break the layout.

This PR adds a targeted CSS rule to ensure Shorts (vertical videos) maintain a proper 9:16 aspect ratio, allowing them to scale correctly within responsive layouts.

Ticket: https://core.trac.wordpress.org/ticket/65067

[github-actions]

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[mukeshpanchal27]

There is no file changes in these PR 🤔

[github-actions]

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[westonruter]

@wprashed you need to update the style.css not the RTL version.

[westonruter]

Also, this seems like it should rather be a style change for the Embed block in the Gutenberg repo, rather than being something theme-specific? Or is there an issue with the Twenty Nineteen theme which is uniquely causing a problem here?