Document MySQL < 8 / MariaDB < 10.3 does not support the referential action onDelete: setDefault

[Jolg42]

Based on prisma/prisma#11498

MySQL < 8 / MariaDB < 10.3 (not 100% sure for that version) does not support the referential action onDelete: setDefault

It errors with Error: Cannot add foreign key constraint.

TODO:

• update https://www.prisma.io/docs/concepts/components/prisma-schema/relations/referential-actions with version incompatibility

[Jolg42]

    I have just found out that `MySQL 8.0` [docs](https://dev.mysql.com/doc/refman/8.0/en/create-table-foreign-keys.html#foreign-key-referential-actions) report the following:

SET DEFAULT: This action is recognized by the MySQL parser, but both InnoDB and NDB reject table definitions containing ON DELETE SET DEFAULT or ON UPDATE SET DEFAULT clauses.

Originally posted by @jkomyno in prisma/prisma#11498 (comment)