Continuing the work to update old dependencies. This updates react-keycloak (deprecated) to @react-keycloak/web, and all other dependencies that seemed unproblematic to update (i.e. they don't require any changes and do not break any tests). Upgrading keycloak required only a few minor changes.
Also resolved some issues with outdated test snapshots, and configure the linter to report an error if using let instead of const.