Add keda alerting rules

[QuentinBisson]

Before adding a new alerting rule into this repository you should consider creating an SLO rules instead.
SLO helps you both increase the quality of your monitoring and reduce the alert noise.

• How to create a SLO rule: https://github.com/giantswarm/sloth-rules#how-to-create-a-slo
• Documentation: https://intranet.giantswarm.io/docs/monitoring/slo-alerting/

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Towards: https://github.com/giantswarm/adidas/issues/1213

This PR adds keda alerting rules. They are configurés to only notify for now

Checklist

• Update CHANGELOG.md
• Add Unit tests
• Follow Alert structure
• Consider creating a dashboard (guidelines) (if it does not exist already) to help oncallers monitor the status of the issue.
• Request review from oncall area, as well as team (e.g: oncall-kaas-cloud GitHub group).

[T-Kukawka]

This should wait for the same reason KSM is waiting, to figure out the whole oncall setup that is currently not working

[QuentinBisson]

Moved ownership to atlas for now until turtles has more time

[hervenicol]

This PR adds keda alerting rules (only KEDA down is a paging alert)

Looks like it is not, as I see it as severity: notify.

Out of curiosity, did you come up with these alerts, or are these alerts suggested upstream? Maybe the link to upstream should be provided as a comment if that's the case.

[QuentinBisson]

So this alert was paging until Yesterday evening 😂
I came up with those myself. They don't provide much upstream apart from exposés metrics