check_if_shop_url_is_reachable fails when shop is in maintenance mode

[MoritzKrafeld]

The configuration option check_if_shop_url_is_reachable fails when the shop is in maintenance mode even though the API of the shop is still available.
The check itself fails because it is being redirected from my-app-url.com to my-app-url.com/maintenance (HTTP 307). Even if we would allow multiple redirects (as this one is only a temporary and not a permanent redirect), it would still fail because the my-app-url.com/maintenance request responds with a HTTP 503 Service Unavailable.
A possible solution could be to check if certain API routes are available and do not return a redirect (3xx) and also no internal server error (5xx).