docs: clarify legacy CI and virtual lab status

[dholt]

Summary

• Clarify that current public PR checks run through GitHub Actions, while Jenkins/Vagrant is legacy/community-supported reference material.
• Update testing docs to require explicit GPU-backed validation notes for changes that affect real deployments.
• Correct the virtual lab docs and startup-script comment so the documented default matches the actual Ubuntu 20.04 default.

Validation

• git diff --check
• bash -n virtual/vagrant_startup.sh
• make public-sanitize PUBLIC_SANITIZE_INPUT=/tmp/deepops-legacy-ci-doc-refresh.diff

Notes:

• This is documentation-only except for a comment typo/default correction in virtual/vagrant_startup.sh.
• No reviewers requested yet; will request after public CI is green.