ValueResolver: revert resolve return type to array

The `array` return type is on purpose. See #17362 (comment)

Author: kaznovac

controller/value_resolver.rst

@@ -219,7 +219,7 @@ values. That's why you must always return an array, even for single values:
 
     class BookingIdValueResolver implements ValueResolverInterface
     {
-        public function resolve(Request $request, ArgumentMetadata $argument): iterable
+        public function resolve(Request $request, ArgumentMetadata $argument): array
         {
             // get the argument type (e.g. BookingId)
             $argumentType = $argument->getType();