AI Client: change AI control shadow depth

[CGastrell]

Adjust AI Assistant shadow for a better sense of depth and decoupling

Fixes #34331

Proposed changes:

This PR changes the shadow to match the designs seen on Figma file: gG1smMHbNGas0qwP9y6RwK-fi-7531%3A1227

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

p1701184924339369-slack-C02TQF5VAJD
p1HpG7-pBB-p2

Does this pull request change what data or activity we track or use?

No

Testing instructions:

Use the link to create a JN instance, go to editor and create an AI Assistant block, see the new shadows match those in designs

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

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  bin/jetpack-downloader test jetpack change/ai-control-shadow-depth
  

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[github-actions]

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[dhasilva]

💯

[dhasilva]

Before	After

The shadow is not visible on themes with black backgrounds, but there is nothing that can be done in that regard.

[ederrengifo]

LGTM!