Set_Plugin_Project - validate JSON via json_validate()

[timse201]

Currently, Set Plugin checks if the API response is valid JSON by looking at the first character ($json && '{' == $json[0]).

If the API response starts with a simple space, a new line, or a hidden character the check fails. It will reject the data, even if the JSON is fine.

fixes: https://meta.trac.wordpress.org/ticket/8218

Replaces the manual string check with PHP 8.3 native json_validate() function.

At present we're running PHP 8.1 on WordPress.org (PHP 8.3 to come shortly).

This check is primarily supposed to catch HTML output from the endpoint, in the case of errors.