[WP 7.1] Background: Fix the legacy gradient UI where a gradient cannot be selected

[ramonjd]

Merge patch for #81056

What

When a theme opts out of the new gradient background in theme.json, the Background panel falls back to the legacy gradient UI. I noticed that selecting a preset there does nothing at all, so this PR fixes it.

Testing

See #81056

[github-actions]

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: andrewserong <andrewserong@git.wordpress.org>

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[github-actions]

Size Change: +7 B (0%)

Total Size: 7.74 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-editor/index.min.js	428 kB	+7 B (0%)

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[andrewserong]

This fixes it for me in the wp/7.1 branch, and the gradient gets saved to the gradient attribute as expected when settings.background.gradient is set to false:

<!-- wp:group {"gradient":"pale-ocean","layout":{"type":"constrained"}} -->

LGTM