Patterns API: Fix the category_slugs format for featured patterns

[ParhamG]

This fixes a regression caused via #704 that returns category_slugs as an object instead of an array for patterns that has the featured category.

This is breaking the Patterns tab in the block editor for all WP sites.

The modified array returned by array_diff() can have non-consecutive indexes, causing it to be JSON-encoded as an object rather than array which is the expected format. Applying array_values() to the response fixes it.

1. View the response for https://api.wordpress.org/patterns/1.0/?page=1&per_page=100&order=desc&orderby=date&locale=en_US&wp-version=6.6.1&pattern-keywords=11
2. Lookup the pattern with "id": 199
3. The value for its "category_slugs" attribute must be an array and not an object

[ryelle]

Thanks for the fix and explanation!