Bug 30490: Adjust constraint itemtypes.itemtypes_ibfk_1

Bug 30490: Adjust constraint itemtypes.itemtypes_ibfk_1

Change from CASCADE to restrict.
In harmony with dbrev 20.06.00.022.

Test plan:
Run the dbrev.
Bonus:
update itemtypes set parent_type='VM' where itemtype='CF';
delete from itemtypes where itemtype='VM';
=> ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`koha_myclone`.`itemtypes`, CONSTRAINT `itemtypes_ibfk_1` FOREIGN KEY (`parent_type`) REFERENCES `itemtypes` (`itemtype`))

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>