Support JSON functions only on MariaDB

[adamchainz]

There are quite a few JSON functions added on MariaDB 10.2+ we should support: https://mariadb.com/kb/en/library/function-differences-between-mariadb-102-and-mysql-57/

[adamchainz]

The list is even longer with MySQL 8: https://mariadb.com/kb/en/function-differences-between-mariadb-102-and-mysql-56/