Use native slack notification for pipeline failure notifications
We are using a CI job to trigger slack notifications on pipeline failures. This is because the official slack notification service could do only either "all branches" or just "default branch" while we need "default branch and protected branches" since we need to know about failures in *-stable
branches also.
gitlab-org/gitlab!16405 (merged) essentially made what we want possible. Now that it is deployed in gitlab.com, we can drop our custom CI job and use the native notification service for pipeline notifications.