Kubernetes: deprecate `namespace` backend config property

[un-def]

Given the existence of kubeconfig's property, the namespace backend configuration property is

• redundant
• misleading: if it's not set, it defaults to default, not contexts[].context.namespace as someone might assume

It's better to phase it out in favor of kubeconfig's property:

• Deprecate (warn, announce in the release notes) but still respect
• Ignore – switch to kubeconfig's namespace property
• Drop from the config schema