[Validator] [Deprecated] add a deprecation note about ElementMetadata

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [yes]
| Tests pass?   | [yes]
| Fixed tickets | [12682]
| License       | MIT
| Doc PR        | []
Please remove the constructor and move the warning below the namespace declaration (see https://github.com/symfony/symfony/pull/12739/files)
please also check when it has been deprecated (this was before 2.7)