Update Keycloak client code to work with latest Keycloak version

As found in a recent [PR](https://github.com/nerc-project/coldfront-plugin-api/pull/49), it was found that our Keycloak client code is currently out-of-date with the latest version of Keycloak. For the current CI to work, we will pin the version of the Keycloak container. We preferably want to remove this pin in the future.