Merge branch '6.0' into 6.1

* 6.0:
  [String] Fix snake conversion
  [DowCrawler] Fix locale-sensitivity of whitespace normalization
  suggest to install the Twig bundle when the required component is already installed
  Update PULL_REQUEST_TEMPLATE.md
  [Serializer] Fix throwing right exception in ArrayDenormalizer with invalid type

UndefinedCallableHandler.php

@@ -11,6 +11,7 @@
 
 namespace Symfony\Bridge\Twig;
 
+use Composer\InstalledVersions;
 use Symfony\Bundle\FullStack;
 use Twig\Error\SyntaxError;
 use Twig\TwigFilter;
@@ -98,6 +99,12 @@ private static function onUndefined(string $name, string $type, string $componen
             return sprintf('Did you forget to %s? Unknown %s "%s".', self::FULL_STACK_ENABLE[$component], $type, $name);
         }
 
-        return sprintf('Did you forget to run "composer require symfony/%s"? Unknown %s "%s".', $component, $type, $name);
+        $missingPackage = 'symfony/'.$component;
+
+        if (class_exists(InstalledVersions::class) && InstalledVersions::isInstalled($missingPackage)) {
+            $missingPackage = 'symfony/twig-bundle';
+        }
+
+        return sprintf('Did you forget to run "composer require %s"? Unknown %s "%s".', $missingPackage, $type, $name);
     }
 }