Update Elytron OIDC client dependency versions to 1.15.9.Final

In upstream, we moved the Elytron OIDC client dependencies from WildFly to Core to avoid needing to keep the version.org.wildfly.security.elytron up to date in both pom.xml files. For XP 4, Carlo noted that we only support one maintenance stream for Elytron (in the given EAP 7.4 + XP 4 combination). With the dependencies moved to Core, we won't be able to fix XP 4 related issues. Thus, he suggested keeping the dependencies in the XP 4 pom and updating to the latest Elytron build.