Bump to Fedora 40

Some of our upstream CIs (ostree, rpm-ostree) require cosa and FCOS to
be on the same release. Ideally we'd fix that but there's details there
and we want to move cosa anyway.

Dockerfile

@@ -1,6 +1,6 @@
 # When rebasing to new Fedora, also update openshift/release:
 # https://github.com/openshift/release/tree/master/ci-operator/config/coreos/coreos-assembler/coreos-coreos-assembler-main.yaml
-FROM quay.io/fedora/fedora:39
+FROM quay.io/fedora/fedora:40
 WORKDIR /root/containerbuild
 
 # Keep this Dockerfile idempotent for local development rebuild use cases.