minor #19966 [Security] Add namespace for AccessTokenHandler (andreybolonin)

This PR was submitted for the 7.1 branch but it was merged into the 6.4 branch instead.

Discussion
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[Security] Add namespace for AccessTokenHandler

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Commits
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9bea798 Update access_token.rst

Author: javiereguiluz

security/access_token.rst

@@ -78,6 +78,7 @@ This handler must implement
     namespace App\Security;
 
     use App\Repository\AccessTokenRepository;
+    use Symfony\Component\Security\Core\Exception\BadCredentialsException;
     use Symfony\Component\Security\Http\AccessToken\AccessTokenHandlerInterface;
     use Symfony\Component\Security\Http\Authenticator\Passport\Badge\UserBadge;