silence-operator release v0.15.0

Migrate from Giant Swarm deprecated operatorkit framework to kube-builder. This change introduces a few breaking changes:
- Operator configuration has been moved from a configmap to command-line arguments. This does not affect helm chart users
- The operator needs new rbac capabilities to be able manage leases and to create events
- http port has been changed from 8000 to 8080.
- Finalizers set on silences have been changed from operatorkit.giantswarm.io/silence-operator-silence-controller to monitoring.giantswarm.io/silence-protection
helm .registry.domain has been renamed to image.registry (breaking change)
helm deprecated PodSecurityPolicy has been removed (breaking change with kubernetes < 1.25)
Use app-build-suite to build the operator.
Changed container image from alpine to a non-root distroless image.

Remove the unnecessary sync job to rely on GitOps. Breaking change: this means that you should now use your favorite GitOps tool (flux, ArgoCD) to deploy silences on your clusters.
Removed Giant Swarm legacy microerrors package for error handling