[Validator] deprecate member metadata accessors

| Q             | A
| ------------- | ---
| Bug fix?      | sort of
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/pull/11614/files#r16385109
| License       | MIT
| Doc PR        | 

deprecate member metadata accessors in favor of existing property metadata accessors
in Symfony 2.4 and lower, ``getConstraints`` is available on the class without it extending GenericMetadata (which does not exist). So you should check for ``\Symfony\Component\Validator\Mapping\ElementMetadata`` as well (otherwise you make the Form guesser of 2.6 incompatible with Validator 2.3 and 2.4 while composer says ``~2.3``)