add versionadded for the data collector form extension
minor #3887 [Components][Form] add versionadded for the data collector form extension (xabbuh)

This PR was merged into the 2.4 branch.

Discussion
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[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.

Commits
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beceed4 add versionadded for the data collector form extension