[Form] solved dependency to ValidatorInterface, fix #11036

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | #11036, #11345
| License       | MIT
| Doc PR        | 

Since Symfony 2.5

The problem was that the form component has a hardcoded decency to the deprecated validator component.
The pull request fixes the dependency to the validator component and supports now both implementations.

@Symfony Core Members
please review the changes https://github.com/sebastianblum/symfony/blob/0a1e9c208f8730219bebf89f6696b246a0c88da7/src/Symfony/Component/Form/Extension/Validator/ValidatorExtension.php
I'm not sure if it was the right solution

should be ``ValidatorInterface|LegacyValidatorInterface``