Update Antora UI Spring to v0.4.18 (#33899)

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Author: github-actions[bot]

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