EntityFormInterface::validate() removed in favor of FormInterface::validateForm()

The EntityFormInterface::validate() validation handler was replaced by the more generic FormInterface::validateForm() method.

The most relevant consequence is that now content entity validation is defined as a form-level handler, which means it will be skipped as soon as button-level validation handler is defined. This in general is highly discouraged, but code needing that will need to manually invoke ContentEntityForm::validateForm(), otherwise an exception will be thrown.