Add aspect ratio for YouTube shorts iframe

[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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Props bappy6652.

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[github-actions]

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