[Validator] added a deprecation note about MetadataFactoryInterface

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12690
| License       | MIT
| Doc PR        | 

Can you use the fully qualified class name? Also, can you put `"` around class names? Thanks.