[Components][Form] add versionadded for the data collector form extension

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4+
| Fixed tickets | 

As @ahsio pointed out in #3886, the data collector extension was added to the Form component in Symfony 2.4.
@javiereguiluz What do think?