Fix deprecations and code style issues

src/Bundle/ChillCalendarBundle/RemoteCalendar/Connector/MSGraph/MSUserAbsenceReader.php

@@ -33,7 +33,7 @@ final readonly class MSUserAbsenceReader implements MSUserAbsenceReaderInterface
     /**
      * @throw UserAbsenceSyncException when the data cannot be reached or is not valid from microsoft
      */
-    public function isUserAbsent(User $user): bool|null
+    public function isUserAbsent(User $user): ?bool
     {
         $id = $this->mapCalendarToUser->getUserId($user);