Use `AlertmanagerConfig` resource instead of CSC ConfigMap

[gdemonet]

Component: alertmanager, csc

Why this is needed: CSC is harder to use, since it isn't reactive and requires a Salt orchestration.

What should be done:

• Remove alertmanager.useExistingSecret and .secrets keys from the kube-prometheus-stack chart configuration, and define an appropriate .alertmanagerSpec.alertmanagerConfigSelector and .alertmanagerConfigNamespaceSelector
• Deploy a default AlertmanagerConfig object (with the same values as today in salt/metalk8s/addons/prometheus-operator/config/alertmanager.yaml)
• Define migration on upgrade from previous CSC to this configuration object
• Rewrite documentation section about Alertmanager configuration

Test plan:

• Post-merge upgrade (and downgrade?) tests pass

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See prometheus-operator/prometheus-operator#3451