fix deprecation of eraseCredentials function return type

Security/User/KeycloakBearerUser.php

@@ -258,8 +258,10 @@ class KeycloakBearerUser implements UserInterface
      *
      * This is important if, at any given point, sensitive information like
      * the plain-text password is stored on this object.
+     * 
+     * @return void
      */
-    public function eraseCredentials()
+    public function eraseCredentials(): void
     {
         // TODO: Implement eraseCredentials() method.
     }