[Form] Fixed ValidatorExtension to work with the 2.5 Validation API
bug #11645 [Form] Fixed ValidatorExtension to work with the 2.5 Validation API (webmozart)

This PR was merged into the 2.5 branch.

Discussion
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[Form] Fixed ValidatorExtension to work with the 2.5 Validation API

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11606
| License       | MIT
| Doc PR        | -

The ValidatorExtension::loadTypeGuesser() method used to fail because the method `getMetadataFactory()` does not exist anymore on the new `ValidatorInterface`. This was fixed easily, because `ValidatorInterface` now extends `ClassMetadataFactory`.

Commits
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ff74f4e [Form] Fixed ValidatorExtension to work with the 2.5 Validation API