minor #14670 [Mercure] Update token generation (flovrent)

This PR was merged into the 5.2 branch.

Discussion
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[Mercure] Update token generation

Following the release of the version 4 of the `lcobucci/jwt` library, we get the following error `Cannot Instantiate Interface Lcobucci\JWT\Builder`, same thing with `Key`.

Like you can see here : https://github.com/lcobucci/jwt/releases/tag/4.0.0, there are BC-break about `Key` and `Builder` who have become Interface.

In accordance with the documentation on https://lcobucci-jwt.readthedocs.io/en/latest/configuration/#configuration, I allow myself to propose the following merge request.

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Commits
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fb8dea5 Update mercure.rst

Author: javiereguiluz

mercure.rst

@@ -408,7 +408,7 @@ And here is the controller::
     // src/Controller/DiscoverController.php
     namespace App\Controller;
 
-    use Lcobucci\JWT\Builder;
+    use Lcobucci\JWT\Configuration;
     use Lcobucci\JWT\Signer\Hmac\Sha256;
     use Lcobucci\JWT\Signer\Key;
     use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
@@ -423,11 +423,14 @@ And here is the controller::
         {
             $hubUrl = $this->getParameter('mercure.default_hub');
             $this->addLink($request, new Link('mercure', $hubUrl));
-
-            $token = (new Builder())
-                // set other appropriate JWT claims, such as an expiration date
-                ->withClaim('mercure', ['subscribe' => ["http://example.com/books/1"]]) // can also be a URI template, or *
-                ->getToken(new Sha256(), new Key($this->getParameter('mercure_secret_key'))); // don't forget to set this parameter! Test value: !ChangeMe!
+            
+            $key = Key\InMemory::plainText('mercure_secret_key'); // don't forget to set this parameter! Test value: !ChangeMe!
+            $configuration = Configuration::forSymmetricSigner(new Sha256(), $key);
+            
+            $token = $configuration->builder()
+                                    ->withClaim('mercure', ['subscribe' => ["http://example.com/books/1"]]) // can also be a URI template, or *
+                                    ->getToken($configuration->signer(), $configuration->signingKey())
+                                    ->toString();
 
             $response = $this->json(['@id' => '/demo/books/1', 'availability' => 'https://schema.org/InStock']);
             $cookie = Cookie::create('mercureAuthorization')