Update bean validation article to use FW8 APIs (#10387)

Suggested by [Jean-Christophe Gueriaud](https://vaadin.com/forum/#!/thread/16876180).

Closes #10386

documentation/articles/UsingBeanValidationToValidateInput.asciidoc

@@ -35,19 +35,18 @@ You can create a field for the name field as you always would:
 [source,java]
 ....
 Person person = new Person("John", 26);
-BeanItem<Person> item = new BeanItem<Person>(person);
+TextField firstName = new TextField("First name");
 
-TextField firstName = new TextField("First name",
-        item.getItemProperty("name"));
-firstName.setImmediate(true);
 setContent(firstName);
 ....
 
-and add the bean validation as a normal validator:
+and bind the field with a bean validation binder:
 
 [source,java]
 ....
-firstName.addValidator(new BeanValidator(Person.class, "name"));
+BeanValidationBinder<Person> binder = new BeanValidationBinder<>(Person.class);
+binder.forField(firstName).bind("name");
+binder.setBean(person);
 ....
 
 Your `firstName` field is now automatically validated based on the