Popover: Add small size prop to close button

[mirka]

In preparation for #65751

What?

Adds size="small" prop to the close Button in the Popover component, visible in mobile viewports when expandOnMobile is true.

Testing Instructions

See the Default Popover story in Storybook. Enable the expandOnMobile prop, and set the view to a narrow viewport.

In the app, this can be seen in the quick inserter:

[github-actions]

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Co-authored-by: mirka <0mirka00@git.wordpress.org>
Co-authored-by: tyxla <tyxla@git.wordpress.org>

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[github-actions]

Flaky tests detected in 7e6feae.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/11591788936
📝 Reported issues:

• [Flaky Test] should be able to open the tags panel and create a new tag #59638 in /test/e2e/specs/editor/various/taxonomies.spec.js

[tyxla]

LGTM 🚀