Update Keycloak example (currently broken)

[manics]

• claim_groups_key is deprecated, replaced by auth_state_groups_key
• manage_groups must be used with the above, otherwise JupyterHub fails to start

[consideRatio]

To fix these docs are an improvement and happy to merge it, but an alternative improvement would be to delete them from z2jh and link to docs like this in oauthenticator instead. I figure that project benefits from a well maintained keycloak example, but that its hard to maintain it in z2jh/tljh/oauthenticator

[manics]

We could make this a very minimal Keycloak example, I've got a CI tested example in https://github.com/manics/zero-to-jupyterhub-k8s-examples/tree/main/keycloak

[manics]

I decided to take out managed groups/roles completely, as it's not very useful unless you also configure Keycloak.