Update Antora Spring UI to v0.4.20

Closes spring-projectsgh-35814

Author: github-actions[bot]

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/v0.4.18/ui-bundle.zip
+    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.20/ui-bundle.zip