remove deprecated app labels for ksm metrics

[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/...

This PR gets rid of the app=kube-state-metrics in favor of the job=kube-state-metrics label as all clusters have upgraded beyond 18.2

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).

[TheoBrigitte]

How do we ensure this does not break any dasboard or alerting rule ?

[QuentinBisson]

Marie tested it a while back and all ksm metric have this job label set

[QuentinBisson]

@TheoBrigitte are you fine with this?