minor #37871 [Validator] mark the AssertingContextualValidator class as internal (xabbuh)

This PR was merged into the 3.4 branch.

Discussion
----------

[Validator] mark the AssertingContextualValidator class as internal

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       |
| License       | MIT
| Doc PR        |

Commits
-------

7ff0287cd6 mark the AssertingContextualValidator class as internal

src/Symfony/Component/Validator/Test/ConstraintValidatorTestCase.php

@@ -341,6 +341,9 @@ class ConstraintViolationAssertion
     }
 }
 
+/**
+ * @internal
+ */
 class AssertingContextualValidator implements ContextualValidatorInterface
 {
     private $expectNoValidate = false;