Technical Story: replace AppEngine Flex by AppEngine Standard for the google_app_engine_flexible_app_version.default_app resource

**Definition of ready**
Ready

**Description**
GAE Flex does not scale to 0, which increases costs unnecessarily when a `default_app` is added. Deploy the `default_app` resource on GAE Standard instead.

**Definition of done**
The default application is deployed on GAE Standard, with the ability to scale to 0
