Update /info/ API endpoints

[johnbillion]

The /info/ API endpoints need to be updated to use JSON encoding.

Previously: #25311

[dd32]

This isn't as simply as switching serialize() to json_encode() (and vice versa) unfortunately.

JSON doesn't support associative arrays, which the API makes use of, all associative arrays are encoded as JSON objects.
PHP's json_decode() has support to decode all objects to associative arrays as a result, but unfortunately the API makes use of both objects and associative arrays, so using that breaks the API response format

Unfortunately this means there's only really a few options:

1. Keep it as PHP serialized data
2. Break compatibility and change the output/input of plugins_api() and themes_api()
3. Add some code to selectively convert some arrays to objects in the response depending on the $args present
4. Use something other than JSON that supports all the PHP datatypes
5. Use our own superset of JSON on top of PHP's superset of JSON (PHP JSON supports more than the JSON standard, such as bools/nulls)

more on #5, it'd be us adding a wrapper around json_decode() and json_encode() and walking over the returned arrays and if a object item is present converting it to an object, something like 25692.layer.diff​

[chriscct7]

The unit test should be moved out to the unit test suite

[johnbillion]

Fixed in [42631].

Closing as a duplicate of #29274.