Widgets: Use array_all() in WP_Widget_Media_Gallery::has_content()

[Soean]

Description

This PR replaces the manual foreach loop in WP_Widget_Media_Gallery::has_content() with the array_all() function, which expresses the same intent — return true only if every attachment ID resolves to a post of type attachment — without the loop-and-early-return boilerplate.

array_all() was introduced in PHP 8.4 and is available in Core for all supported PHP versions via the polyfill in wp-includes/compat.php (since 6.8.0).

The surrounding conditional is also flattened into an early return, but that is a minor side effect. No functional change.

Trac ticket:

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