minor #13568 [Validator] use 2.5 API in LengthValidator (nicolas-grekas)

This PR was merged into the 2.5 branch.

Discussion
----------

[Validator] use 2.5 API in LengthValidator

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

Commits
-------

7602454 [Validator] use 2.5 API in LengthValidator

src/Symfony/Component/Validator/Constraints/LengthValidator.php

@@ -64,10 +64,11 @@ class LengthValidator extends ConstraintValidator
         }
 
         if ($invalidCharset) {
-            $this->context->addViolation($constraint->charsetMessage, array(
-                '{{ value }}' => $this->formatValue($stringValue),
-                '{{ charset }}' => $constraint->charset,
-            ), $value);
+            $this->buildViolation($constraint->charsetMessage)
+                ->setParameter('{{ value }}', $this->formatValue($stringValue))
+                ->setParameter('{{ charset }}', $constraint->charset)
+                ->setInvalidValue($value)
+                ->addViolation();
 
             return;
         }