[Validator] Add deprecation message about Symfony\Component\Validator\MetadataInterface.
minor #12739 [Validator] Add deprecation message #12677. (xelaris)

This PR was merged into the 2.7 branch.

Discussion
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[Validator] Add deprecation message #12677.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12677
| License       | MIT
| Doc PR        |

Commits
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a00c954 [Validator] Add deprecation message about Symfony\Component\Validator\MetadataInterface.