Add slack notification to k8s-discoverity-test workflow

[reakaleek]

What does this PR do?

Add notification to k8s-autodiscovery-test workflow

Why is it important?

So we are notified about failures.

[mergify]

This pull request does not have a backport label. Could you fix it @reakaleek? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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• Start Time: 2023-08-02T12:09:58.491+0000

• Duration: 2 min 5 sec

❕ Flaky test report

No test was executed to be analysed.

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