Use GitHub action to raise Antora UI version upgrade issue

[philwebb]

https://github.com/spring-io/spring-doc-actions?tab=readme-ov-file#update-antora-ui-spring

[wilkinsona]

I'm not sure this is really worth it. AIUI, the action will create a PR which we'd then have to merge. It looks like the PR title and commit message won't match our style so we'd probably end up editing them. I think it'd be just as easy, if not easier, to watch the repo for releases and then do the upgrade manually.

[philwebb]

Yeah, I agree. I wonder if we can rip out the logic to tell if there's an update and raise an issue so we don't need to manually watch the repo.