minor #10058 Remove deleted RoleInterface (Aluok, javiereguiluz)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #10058).

Discussion
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Remove deleted RoleInterface

As the master branch of symfony/symfony deleted the Symfony\Component\Security\Core\Role\RoleInterface as of 4.0, this update remove the interface to only mention the base class.

Commits
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c410eb0 Reword
a494993 Remove deleted RoleInterface

Author: javiereguiluz

components/security/authorization.rst

@@ -167,12 +167,11 @@ role::
 Roles
 -----
 
-Roles are objects that give expression to a certain right the user has.
-The only requirement is that they implement :class:`Symfony\\Component\\Security\\Core\\Role\\RoleInterface`,
-which means they should also have a :method:`Symfony\\Component\\Security\\Core\\Role\\RoleInterface::getRole`
-method that returns a string representation of the role itself. The default
-:class:`Symfony\\Component\\Security\\Core\\Role\\Role` simply returns its
-first constructor argument::
+Roles are objects that give expression to a certain right the user has. The only
+requirement is that they must define a ``getRole()`` method that returns a
+string representation of the role itself. To do so, you can optionally extend
+from the default :class:`Symfony\\Component\\Security\\Core\\Role\\Role` class,
+which returns its first constructor argument in this method::
 
     use Symfony\Component\Security\Core\Role\Role;