[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        | []
This should mention which class has been deprecated.