Update Antora Spring UI to v0.4.15

Author: github-actions[bot]

antora-playbook.yml

@@ -36,5 +36,5 @@ runtime:
     failure_level: warn
 ui:
   bundle:
-    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.3/ui-bundle.zip
+    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.15/ui-bundle.zip
     snapshot: true