Update Antora Spring UI to v0.4.26

Author: github-actions[bot]

spring-grpc-docs/src/main/antora/antora-playbook.yml

@@ -34,5 +34,5 @@ runtime:
     format: pretty
 ui:
   bundle:
-    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.3.6/ui-bundle.zip
+    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.26/ui-bundle.zip
     snapshot: true