Workspaces: Negate the hostUsers boolean

What does this MR do and why?

In Workspaces, if I want to use Kubernetes User Namespaces (available in GKE from 1.33) I need to set

remote_development:
  # NOTE: This is a partial example.
  # Some required fields are not included.
  use_kubernetes_user_namespaces: true

However, this passes the true value directly through to the deployment/pod spec. And when we don't have a true we don't pass that through into the deployment/pod at all!

Whereas the kubernetes implementation needs a false there (in hostUsers) to 'enable' user namespaces:

apiVersion: v1
kind: Pod
metadata:
  name: userns
spec:
  hostUsers: false
  containers:
  - name: shell
    command: ["sleep", "infinity"]
    image: debian

This MR reverses the value we set, so that we correctly set false when we want to enable user namespaces.

References

https://kubernetes.io/docs/tasks/configure-pod-container/user-namespaces/

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