block automatic major version upgrade with annotation for single cluster

charts/postgres-operator/crds/operatorconfigurations.yaml

charts/postgres-operator/values.yaml

docs/reference/operator_parameters.md

manifests/configmap.yaml

manifests/operatorconfiguration.crd.yaml

manifests/postgresql-operator-default-configuration.yaml

pkg/apis/acid.zalan.do/v1/crds.go

pkg/apis/acid.zalan.do/v1/operator_configuration_type.go

pkg/cluster/majorversionupgrade.go

pkg/controller/operator_config.go

pkg/util/config/config.go