Make usable with PHP<8.0

As the library is set to be usable with PHP7.4 and the
ReflectionProperty::has/getDefaultValue is only available for PHP8+ I
needed to slightly refactor the class.

src/ModelDescriber/Annotations/ReflectionReader.php

@@ -43,9 +43,6 @@ public function updateProperty(
         OA\Property $property,
         ?array $validationGroups = null
     ): void {
-        if (PHP_VERSION_ID < 80000) {
-            return;
-        }
         // The default has been set by an Annotation or Attribute
         // We leave that as it is!
         if (Generator::UNDEFINED !== $property->default) {
@@ -136,15 +133,12 @@ public function getDefaultFromPropertyReflection(\ReflectionProperty $reflection
             return Generator::UNDEFINED;
         }
 
-        $reflectionProp = $reflection->getDeclaringClass()->getProperty($propertyName);
-        if (!$reflectionProp->hasDefaultValue()) {
-            return Generator::UNDEFINED;
-        }
+        $defaultValue = $reflection->getDeclaringClass()->getDefaultProperties()[$propertyName]??null;
 
-        if (null === $reflectionProp->getDefaultValue()) {
+        if (null === $defaultValue) {
             return Generator::UNDEFINED;
         }
 
-        return $reflectionProp->getDefaultValue();
+        return $defaultValue;
     }
 }