tf-stage2: update keycloak provider to 3.8.1

OpenID clients:
- 'use_refresh_tokens' set to false to preserve the values on live
- 'backchannel_logout_session_required' implicitly changed to true
  for the 'grafana_openid_client' and 'openid_gitlab' clients

SAML client (GitLab):
- 'front_channel_logout' set to false to preserve the live setting