Update "Validating Values and Arrays" section of book to use non-deprecated methods

[stevenmusumeche]

It is currently showing how to use validateValue which is deprecated.

[xabbuh]

I think we can add this to the task list in #4094. What do you think?

[stevenmusumeche]

I agree, because the behavior is different when using the new ValidationInterface even though the docs says it is just renamed.

[xabbuh]

@stevenmusumeche I added this to the todo list in #4094. Thank you for reporting it!