[8.x] Revert isDownForMaintenance function to use file_exists()

[codywoolsey]

After updating one of our applications to Laravel 8.x, we noticed a recurring issue with our queue workers after deployments. When we would run php artisan queue:restart during the deployment process, the worker processes would restart as expected, but they would no longer work jobs from the queue.

After some investigation, we determined this issue was specifically occurring because our deployment script executes php artisan queue:restart while the application is still in maintenance mode. It appeared that even after we brought the application out of maintenance mode, the isDownForMaintenance check from within the worker processes was still returning true.

It appears the issue was introduced in this PR, which would explain why we never encountered this issue prior to updating to Laravel 8.x. We believe the issue is related to differences in how the statcache is utilized between is_file() and file_exists().

Steps to reproduce issue

1. Setup a supervised process to run the php artisan queue:work command.
2. Start the queue worker.
3. Run php artisan down to bring the application into maintenance mode.
4. Run php artisan queue:restart to restart the queue worker.
5. Run php artisan up to bring the application out of maintenance mode.
6. Dispatch a job onto the worker's queue. The job will never be processed.

[driesvints]

Ping @GrahamCampbell

[GrahamCampbell]

Fine by me. Should go to 6.x if relevant.