Ensure all registry scheduled job have slack alerts for failures

[thepetk]

Which area is this issue related to?

/area ci
/area registry

Issue Description

We should make sure that all the scheduled jobs of the registry include a slack notification step for every failure.

This way we will get notified if a job fails and try to find a fix.

At the moment only the validate-stacks has a notification in place. Ideally we should have one in validate-samples too.

Another part of the task is to confirm that the current notification steps are working and we are still receiving alerts on slack if something fails.

Acceptance Criteria

• validate-samples has a slack notification step
• all slack notification steps in registry workflows work fine.

Target Odo Version

Target Odo version:

[github-actions]

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