FocalPointPicker: Complete __next40pxDefaultSize cleanup

[mirka]

What?

Follow up to #65751 and #64456.

FocalPointPicker already unconditionally renders 40px child controls. This PR finishes the remaining bookkeeping: runtime prop stripping and ESLint.

Why?

The public __next40pxDefaultSize prop was deprecated in WP 6.7 and has been a noop since #64456 defaulted the component to 40px internally. This aligns lint rules with that reality and prevents the prop from leaking onto the root element via ...restProps.

How?

• Destructure __next40pxDefaultSize in FocalPointPicker so it is not spread onto the root Container.
• Add a restricted-syntax rule to forbid passing __next40pxDefaultSize to FocalPointPicker.

No runtime behavior change.

Testing Instructions

1. Open Storybook to Components / FocalPointPicker and smoke test the default story.
2. Drag the focal point and adjust the Left/Top unit controls.
3. Insert a Cover block with a background image and confirm the focal point picker in the sidebar still renders at 40px height.
4. Confirm no console warnings are logged.

[github-actions]

Size Change: +42 B (0%)

Total Size: 7.51 MB

📦 View Changed

Filename	Size	Change
build/scripts/components/index.min.js	264 kB	+42 B (+0.02%)

_{compressed-size-action}

[github-actions]

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

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[ciampo]

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