Set_Plugin_Project - validate JSON via json_validate()

[timse201]

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

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

[github-actions]

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Props timse201, dd32.

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[dd32]

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.