Retire legacy PXE provisioning paths

[dholt]

Summary

Retires DeepOps-maintained legacy PXE/preseed provisioning paths in favor of MAAS, existing site provisioning systems, or Ubuntu autoinstall/cloud-init for current Ubuntu 24.04 and DGX OS 7 deployments.

This removes:

• DGXIE docs, container, Helm chart, scripts, and example config.
• The old minimal PXE/Pixiecore container path and its Ubuntu 16.04/18.04 preseed assets.
• Stale Kubernetes examples used only for DGX ISO/PXE loading.

It also updates MAAS guidance to keep DeepOps modular and simple:

• MAAS remains the source of truth for machine lifecycle, power, DHCP/PXE, images, commissioning, deployment, release, pools, zones, and tags.
• DeepOps consumes MAAS via tags, dynamic inventory, explicit deploy/status/release helper scripts, and Ansible validation artifacts.
• DeepOps should not run a second provisioning reconciler or compete with BCM.

MAAS update

• Defaults MAAS controllers on Ubuntu 24.04+ to ppa:maas/3.7.
• Keeps Ubuntu 22.04 controllers on ppa:maas/3.5.
• Refreshes the Noble airgap mirror example to the MAAS 3.7 PPA.

Validation

• git diff --check
• YAML parse for:
  • config.example/group_vars/all.yml
  • playbooks/provisioning/maas.yml
  • config.example/maas-inventory.yml
• python3 -m py_compile scripts/maas_inventory.py
• bash -n scripts/maas_deploy.sh
• ansible-playbook -i localhost, -c local playbooks/provisioning/maas.yml --syntax-check with the pinned ansible-maas Galaxy role available in ANSIBLE_ROLES_PATH
• make os-compat-audit RELEASE=26.07 WORKTREE=<checked-out branch>
  • Result after cleanup: {"high": 8, "medium": 33, "low": 77}
  • Remaining high findings are legacy container-runtime docs/package references, not PXE/provisioning.

Notes

This is intentionally a cleanup/scope PR. It does not add a new state manager; the goal is to keep DeepOps accessible and contributor-friendly while letting MAAS own provisioning state.