fix: Unknown Constraint class when not using symfony/validator (#2276)

| Q | A |

|---------------|---------------------------------------------------------------------------------------------------------------------------|
| Bug fix? | yes |
| New feature? | no <!-- please update src/**/CHANGELOG.md files --> |
| Deprecations? | no <!-- please update UPGRADE-*.md and
src/**/CHANGELOG.md files --> |
| Issues | Fix #2275 <!-- prefix each issue number with "Fix #", no need
to create an issue if none exists, explain below instead --> |

#2259 accidentally
removed the `class_exists` check for symfony/validator `Constraint`.
This currently causes applications using this bundle to fail because of
the missing symfony/validator dependency

src/ModelDescriber/Annotations/SymfonyConstraintAnnotationReader.php

@@ -213,7 +213,7 @@ private function getAnnotations(Context $parentContext, $reflection, ?array $val
      */
     private function locateAnnotations($reflection): \Traversable
     {
-        if (\PHP_VERSION_ID >= 80000) {
+        if (\PHP_VERSION_ID >= 80000 && class_exists(Constraint::class)) {
             foreach ($reflection->getAttributes(Constraint::class, \ReflectionAttribute::IS_INSTANCEOF) as $attribute) {
                 yield $attribute->newInstance();
             }