Upgrade to antora-ui-spring version 0.4.18

Closes gh-33892

Author: sbrannen

framework-docs/antora-playbook.yml

@@ -36,4 +36,4 @@ runtime:
     failure_level: warn
 ui:
   bundle:
-    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.17/ui-bundle.zip
+    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.18/ui-bundle.zip