Update Antora UI bundle to v0.4.26 (#1027)

Signed-off-by: spring-builds <spring-builds@users.noreply.github.com>
Co-authored-by: spring-builds <spring-builds@users.noreply.github.com>

Author: spring-builds

docs/antora-playbook.yml

@@ -30,4 +30,4 @@ runtime:
     format: pretty
 ui:
   bundle:
-    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.15/ui-bundle.zip
+    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.26/ui-bundle.zip