[release/4][BACKPORT] [prometheus] Bump kube-prometheus-stack to 15.4.9

[d2iq-mergebot]

This is a backport of the following PR:

#1329

What type of PR is this?

bug

What this PR does/ why we need it:

mesosphere/charts#1234
crd-alertmanager.yaml was renamed to crd-alertmanagers.yaml in chart version v11.0.0, but we didn't catch this change in any of our upgrade scripts so any users on v11+ will run into outdated alertmanager CRDs upon upgrading. I traced this back to any version including and over konvoy 1.6.2.

Which issue(s) this PR fixes:

https://jira.d2iq.com/browse/COPS-6842
https://jira.d2iq.com/browse/D2IQ-79666

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Alertmanager CRD is now properly upgraded (COPS-6842)

Checklist

• The commit message explains the changes and why are needed.
• The code builds and passes lint/style checks locally.
• The relevant subset of integration tests pass locally.
• The core changes are covered by tests.
• The documentation is updated where needed.

[d2iq-mergebot]

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