Use 'Extension' under 'DependencyInjection' component

The old "Symfony\Component\HttpKernel\DependencyInjection\Extension" class is
considered internal since Symfony 7.1, to be deprecated in 8.1. According to
the official deprecation message, we should use
Symfony\Component\DependencyInjection\Extension\Extension instead.

Fixed #98

src/DependencyInjection/MjmlExtension.php

@@ -7,8 +7,8 @@
 use Symfony\Component\Config\FileLocator;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Definition;
+use Symfony\Component\DependencyInjection\Extension\Extension;
 use Symfony\Component\DependencyInjection\Loader\YamlFileLoader;
-use Symfony\Component\HttpKernel\DependencyInjection\Extension;
 
 class MjmlExtension extends Extension
 {