Fix registration of backchannel logout route

Currently the backchannel logout route is registered with the root-path, resulting in a path like this:
``/backend/backend/back-channel-logout`` instead of ``/backend/back-channel-logout``

See also quarkusio#42990

(cherry picked from commit f92fc56)

extensions/oidc/runtime/src/main/java/io/quarkus/oidc/runtime/BackChannelLogoutHandler.java

@@ -46,7 +46,7 @@ public void setup(@Observes Router router) {
 
     private void addRoute(Router router, OidcTenantConfig oidcTenantConfig) {
         if (oidcTenantConfig.isTenantEnabled() && oidcTenantConfig.logout.backchannel.path.isPresent()) {
-            router.route(getRootPath() + oidcTenantConfig.logout.backchannel.path.get())
+            router.route(oidcTenantConfig.logout.backchannel.path.get())
                     .handler(new RouteHandler(oidcTenantConfig));
         }
     }