Dynamic (non-explicitly declared) properties are deprecated as of PHP 8.2 and are expected to become a fatal error in PHP 9.0.
In this case, the `$plugin_info` and `$theme_info` properties are set in `Plugin_Upgrader::bulk_upgrade()` and `Theme_Upgrader::bulk_upgrade()` specifically.
The `Bulk_Plugin_Upgrader_Skin` class and the `Bulk_Theme_Upgrader_Skin` class both already allow for this, but the `wp_ajax_update_plugin()` and `wp_ajax_update_theme()` functions also call the `*_Upgrader::bulk_upgrade()` methods, so the `WP_Ajax_Upgrader_Skin` class also needs to have these properties explicitly declared.
Includes adding proper DocBlocks for the pre-existing properties in the `Bulk_Plugin_Upgrader_Skin` and the `Bulk_Theme_Upgrader_Skin` classes.
Follow-up to [13686], [37714], [38199], [42677], [42873], [53557], [53558], [53850], [53851], [53852], [53853], [53854], [53856], [53916], [53935], [53936], [53937], [53938], [53942], [53945], [53948], [53949].
Props jrf, costdev.
See #56033.
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