[Validator] [Hackday] [2.7]  Add a deprecation note about PropertyMetadataContainerInterface

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12679 
| License       | MIT
| Doc PR        |
shouldn't this become The "Symfony\Component\Validator\PropertyMetadataContainerInterface" Interface is ... i saw this a few times ago.