Atlassian Bamboo CI integration no longer working (Bamboo not called by Gitlab)

Summary

Atlassian Bamboo CI integration no longer working (Bamboo not called by Gitlab)

Steps to reproduce

1. Navigate to the Integrations page of a Git project
2. Click Atlassian Bamboo CI in the Project services section
3. Existing settings on that page that were working before, or any new configuration are no longer working. For example, setting whatever password or url and click "Test settings and save changes" won't be validated by Gitlab (Gitlab-rail's log on the Gitlab server does not show any sign that Gitlab tried to call Bamboo on that url that it used to). Essentially, any invalid url or username/password is always accepted by Gitlab without initial validation (Test settings) and this hook won't get called upon real git commits/pushes.

Example Project

My own test project on gitlab.com here is also not working.

What is the current bug behavior?

Nothing - no calls are made by Gitlab opposed to what it is supposed to do.

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

Happens on my instance, as well as gitlab.com instance

It seems also affecting other Webhooks as well. Another reported case is https://gitlab.com/gitlab-org/gitlab-ce/issues/61600