As a follow up of this issue  MDL-79895

PHP 8.3 adds a new function named json_validate that returns true or false whether the given string is a valid JSON string. Here more information about it https://php.watch/versions/8.3/json_validate

The task for this ticket will be: 

• Check the possibilities to use the new json_validate in places where we currently perform poor-man's validation
• Create a polyfill, with tests, and make use of it in a few places. Document its existence, and create a todo to remove it when our minimum version is 8.3. 
• This task will be done only for master.