Use doctrine type of "json_array" for JSON columns

[maddhatter]

Doctrine doesn't have a JSON type, it calls it JSON_ARRAY.

Currently, trying to change() a JSON column results in this error:

[Doctrine\DBAL\DBALException]
  Unknown column type "json" requested. Any Doctrine type that you use has to be registered with \Doctrine\DBAL\Types\Type::addType(). You can get a list of all the known types with \Doctrine\DBAL\Types\Type::getTypesMap(). If this er
  ror occurs during database introspection then you might have forgot to register all database types for a Doctrine Type. Use AbstractPlatform#registerDoctrineTypeMapping() or have your custom types implement Type#getMappedDatabaseTyp
  es(). If the type name is empty you might have a problem with the cache or forgot some mapping information.

This change resolves the issue.

[taylorotwell]

Please send to 5.4 branch.