Add missing response to `/xb/api/publish-all` in `openapi.yml`

@tedbow, for some reason I can't seem to create an MR. The button's just absent from the UI. (I feel like we've had a problem with this before, and it might've been a permissions issue.) In any case, I've pushed a branch with the fix. Could you see if you're able to create an MR and then review it?

🤔 MR button is gone for me too!

wim leers → credited larowlan → .

A change in this file cannot possibly cause E2E test failures. IOW: the CI failure is a random failure. Let's wrap this up :)

Thanks for your watchful eye, @traviscarden! 👏🙏