Resolve "QA scheduled master pipeline failure notifications are not being sent to Slack"

What does this MR do?

Notify slack when scheduled:package-and-qa fails

With allow_failure: true on a prerequisite job, when the job fails it would be considered a success. This results in the schedule:package-and-qa job being skipped.

To work around this, we always run schedule:package-and-qa:notify-failure, and look for a failed schedule:package-and-qa in the same pipeline.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Closes #38044 (closed)