Migrate data property of ContentNode and child classes to support Doctrine ORM 3.0

[BacLuc]

doctrine/orm#10455

If we activate this switch, the api service fails to start because the
public data property of ContentNode is overridden in the children.
See https://github.com/BacLuc/ecamp3/tree/report_fields_where_declared

Idea:
Use setters for the additional validation, and getters for the api_platform openapi documentation
but don't redeclare the doctrine orm column $data