Update FindAndModifyCommandSubscriber.php

Using $model::firstOrCreate(...)
currently results in deprecation warnings.  
Can be addressed by adding `void` return type.

Warnings: 
```
   DEPRECATED  Return type of MongoDB\Laravel\Internal\FindAndModifyCommandSubscriber::commandStarted(MongoDB\Driver\Monitoring\CommandStartedEvent $event) should either be compatible with MongoDB\Driver\Monitoring\CommandSubscriber::commandStarted(MongoDB\Driver\Monitoring\CommandStartedEvent $event): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in vendor/mongodb/laravel-mongodb/src/Internal/FindAndModifyCommandSubscriber.php on line 26.


   DEPRECATED  Return type of MongoDB\Laravel\Internal\FindAndModifyCommandSubscriber::commandSucceeded(MongoDB\Driver\Monitoring\CommandSucceededEvent $event) should either be compatible with MongoDB\Driver\Monitoring\CommandSubscriber::commandSucceeded(MongoDB\Driver\Monitoring\CommandSucceededEvent $event): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in vendor/mongodb/laravel-mongodb/src/Internal/FindAndModifyCommandSubscriber.php on line 30.


   DEPRECATED  Return type of MongoDB\Laravel\Internal\FindAndModifyCommandSubscriber::commandFailed(MongoDB\Driver\Monitoring\CommandFailedEvent $event) should either be compatible with MongoDB\Driver\Monitoring\CommandSubscriber::commandFailed(MongoDB\Driver\Monitoring\CommandFailedEvent $event): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in vendor/mongodb/laravel-mongodb/src/Internal/FindAndModifyCommandSubscriber.php on line 22.
```

Author: wivaku

src/Internal/FindAndModifyCommandSubscriber.php

@@ -19,15 +19,15 @@ final class FindAndModifyCommandSubscriber implements CommandSubscriber
 {
     public bool $created;
 
-    public function commandFailed(CommandFailedEvent $event)
+    public function commandFailed(CommandFailedEvent $event): void
     {
     }
 
-    public function commandStarted(CommandStartedEvent $event)
+    public function commandStarted(CommandStartedEvent $event): void
     {
     }
 
-    public function commandSucceeded(CommandSucceededEvent $event)
+    public function commandSucceeded(CommandSucceededEvent $event): void
     {
         $this->created = ! $event->getReply()->lastErrorObject->updatedExisting;
     }