Update return type to Container

With the update of Symfony 7.0, the return type is changed in `KernelTestCase`,
resulting in:
```
Return type declaration must be compatible with KernelTestCase::getContainer() : \Symfony\Component\DependencyInjection\Container
```

tests/Functional/TestCase.php

@@ -13,7 +13,7 @@
 
 use Overblog\DataLoaderBundle\Tests\Functional\app\AppKernel;
 use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;
-use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\DependencyInjection\Container;
 use Symfony\Component\Filesystem\Filesystem;
 use Symfony\Component\HttpKernel\KernelInterface;
 
@@ -49,7 +49,7 @@ public static function setUpBeforeClass(): void
         $fs->remove(sys_get_temp_dir().'/OverblogDataLoaderBundle/');
     }
 
-    protected static function getContainer(): ContainerInterface
+    protected static function getContainer(): Container
     {
         return static::$kernel->getContainer();
     }