Update serializable interface

This should prepare for the future deprecation of the `Serializable` interface.

src/Entity/User.php

@@ -247,16 +247,26 @@ public function eraseCredentials(): void
      */
     public function serialize(): string
     {
-        return serialize([$this->id, $this->username, $this->password]);
+        return serialize($this->__serialize());
+    }
+
+    public function __serialize(): array
+    {
+        return [$this->id, $this->username, $this->password];
     }
 
     /**
      * {@inheritdoc}
      */
     public function unserialize($serialized): void
+    {
+        $this->__unserialize(unserialize($serialized, ['allowed_classes' => false]));
+    }
+
+    public function __unserialize(array $data): void
     {
         // add $this->salt too if you don't use Bcrypt or Argon2i
-        [$this->id, $this->username, $this->password] = unserialize($serialized, ['allowed_classes' => false]);
+        [$this->id, $this->username, $this->password] = $data;
     }
 
     public function getLastseenAt(): ?\DateTimeInterface