[PHP version] Default should be .config.platform.php

[EtienneM]

Configuring PHP version in .config.platform.php is the way we highlight in our doc: https://doc.scalingo.com/languages/php/start#select-a-version

Hence, even though we support multiple ways to declare the PHP versions, we should first use the version declared in .config.platform.php. Otherwise it's confusing for the customer.

https://app.intercom.io/a/apps/w4oogu7s/inbox/inbox/all/conversations/25994170525