Org.keycloak:keycloak-quarkus-server: unrestricted admin use of system and environment variables

A vulnerability was found in Keycloak. Admin users may have to access sensitive server environment variables and system properties through user-configurable URLs. When configuring backchannel logout URLs or admin URLs, admin users can include placeholders like ${env.VARNAME} or ${PROPNAME}. The server replaces these placeholders with the actual values of environment variables or system properties during URL processing.