Update Antora Spring UI to v0.4.15

Author: github-actions[bot]

antora-playbook.yml

@@ -51,7 +51,7 @@ urls:
   redirect_facility: httpd
 ui:
   bundle:
-    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.11/ui-bundle.zip
+    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.15/ui-bundle.zip
     snapshot: true
 runtime:
   log: