minor #17653 Update getContainerExtension() (raphaelriehl)

This PR was merged into the 6.0 branch.

Discussion
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Update getContainerExtension()

Since Symfony 6.x call to getContainerExtension() has evolved. Now, it's necessary to set a return type, else it generates this error :
```php
Declaration of Acme\TestBundle\AcmeTestBundle::getContainerExtension() must be compatible with Symfony\Component\HttpKernel\Bundle\Bundle::getContainerExtension(): ?Symfony\Component\DependencyInjection\Extension\ExtensionInterface
```

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Commits
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8de38ef Update getContainerExtension()

Author: javiereguiluz

bundles/extension.rst

@@ -54,10 +54,11 @@ method to return the instance of the extension::
 
     // ...
     use Acme\HelloBundle\DependencyInjection\UnconventionalExtensionClass;
+    use Symfony\Component\DependencyInjection\Extension\ExtensionInterface;
 
     class AcmeHelloBundle extends Bundle
     {
-        public function getContainerExtension()
+        public function getContainerExtension(): ?ExtensionInterface
         {
             return new UnconventionalExtensionClass();
         }