[2.7] adds deprecation notices.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

@hhamon this class is used in Symfony to provide the 2.5 and 2.4 APIs at the same time. this is the 2.5-BC API. The goal of this class is to implement BC layers on top of the new API. As said previously, the only reason why it is in a child class rather than in the Validator class itself is because implementing both validator interfaces together requires PHP 5.3.9+.

So to avoid triggering a deprecation warning for all Symfony users, warnings should be triggered only in the BC layers of this class, not when using the class (and actually, it is already done, except that the warning is missing in ``getMetadataFactory``).