Describe the bug

An existing Latest Posts block that shows posts from a category no longer filters the category, it shows all posts. Previous GB versions saved the category as a string, but the change in #20781 switched this to an array.

To reproduce

1. Using Gutenberg 7.7.1 (or other previous version), add a Latest Posts block with a selected category
2. Update to master, view your post — it's no longer respecting the category restriction
3. On the frontend, you get a PHP warning: array_column() expects parameter 1 to be array, string given in /var/www/html/wp-content/plugins/gutenberg/build/block-library/blocks/latest-posts.php on line 50
4. Switch into the editor, your category is no longer selected (and the attribute is removed, since it's invalid)