Change update/release cadence to once every other week

We're currently building and releasing a new version of Silverblue every day and shipping that directly to users, before we even run OpenQA tests on it.

The proposal is to change this to a "every other week" release cadence to let time to test things before they are released to all users.

We would also keep a ref/stream/branch that behaves just like what we have right now (daily builds immediately released) for users that want faster updates and for general testing. This would also let us do update tests from latest stable to latest build.

This would be similar in spirit to what Fedora CoreOS does.

Previous discussion in https://github.com/fedora-silverblue/issue-tracker/issues/384.

This requires us to move to bootable containers first as that should make this more manageable / doable in the Fedora infra: #2