bundle.yaml file for the current kubernetes-core bundle lists Focal as the default series, but that can be overridden with a bundle overlay if desired. The charms have all supported Bionic for the past 2 years and Bionic was the default in all of the bundles up until the 1.19 release at which point it was changed to Focal (but Bionic and Xenial are still supported), so you should be fine with any version of the kubernetes-core bundle (though I’d generally recommend just using the latest bundle and setting the
channel config option on the charms if you need an older Kubernetes release for some reason; LMK if you’d like an example of how to do that).
As for the CLI, the easiest way to tell what series a given charm supports is with
charm show (
snap install charm --classic if you don’t already have that tool), which can be given a charm URL with or without a specific revision (or you can use a channel). This doesn’t tell you the default series for a bundle, unless you use the
--all flag, in which case it’s included under the
bundle-metadata section (which will also list the revisions of all of the charms included in the bundle, in case you want to dig in further from there).