Add type hint to FixturesLoadCommand

Fix error phpstan

  Line   Command/FixturesLoadCommand.php
 ------ -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  112    Return type mixed of method Propel\Bundle\PropelBundle\Command\FixturesLoadCommand::execute() is not covariant with return type int of method Symfony\Component\Console\Command\Command::execute().

Command/FixturesLoadCommand.php

@@ -109,7 +109,7 @@ protected function configure(): void
     /**
      * {@inheritdoc}
      */
-    protected function execute(InputInterface $input, OutputInterface $output)
+    protected function execute(InputInterface $input, OutputInterface $output): int
     {
         $this->filesystem = new Filesystem();