containers/ws: Move to Fedora 40

[martinpitt]

Tested locally with

podman build -t cockpit/ws:local containers/ws
podman run -it -p 9999:9090 --rm localhost/cockpit/ws:local

and logging into my laptop and my server on https://localhost:9999

This isn't covered by our CI directly, as it doesn't download the base image, but instead uses the existing container on our fedora-coreos image.

[mvollmer]

Thanks!