keycloak: temporarily allow logout w/ redirect_uri

Keycloak 18.0.0 disallows this by default; enable the legacy behavior
temporarily. When this stops working, we should consider removing the
'redirect_uri' parameter entirely. Should also check if GitLab and/or
Grafafa have implemented support for alternative ways of signing out:

- https://gitlab.com/gitlab-org/gitlab/-/issues/14414
- https://github.com/grafana/grafana/issues/24643