Save DateTime objects in UTC

What?
- Datetime fields
- Standard fields (createdDate, updatedDate)

How?
- [Example](https://github.com/zikula/core/blob/main/src/system/UsersBundle/src/Entity/User.php#L77)
- Also update constraints (add timezone to e.g. `LessThan`, see [docs](https://symfony.com/doc/current/reference/constraints/LessThan.html#comparing-dates))
