Support for Ubuntu releases newer than Eoan

Setting `series` key in `bundle.yaml` file to Ubuntu releases newer than Eoan, results in an error. I've submitted canonical/rust-libjuju#5 for this, but when that is merged, the `juju-bundle` needs to be updated and releases as well.

Start with bundle.yaml and inspect how the series key is handled for Ubuntu releases newer than Eoan. Check canonical/rust-libjuju#5 first, since the issue says this depends on that change. Done means updating juju-bundle after the dependency merges and releasing the updated plugin.

Written by the indexing model from the issue text.